Sometimes, a reminder that even though bad things are happening in your life that God loves you is what you may need to hear or "see". I find it a stretch that because someone is terminally ill that seeing the words "God loves you" would cause them more pain. I could see if it said other things that it might be upsetting but surely people who are terminally ill know death is a part of life and just because they are sick does not mean God does not love them. God didn't cause the sickness--believers in God and non-believers both get sick and die.
I think a positive message that God loves can't be a bad thing and I think most non-believers in God are probably OK with that being written as opposed to "Repent now" or something similar. Most non-believers seem to be pretty open-minded, respectful people, imho. They may finding it annoying but really, I doubt they are truly put off by it.
Writing God loves you in the air is hardly "preaching at" someone. Knowing God loves you is a comforting thought for many and has nothing to do with believeing in Jesus as the savior of the world. It just isn't about a Christian preaching or showing "the way" to non-believers of Jesus. It is proclaiming God's love to all.
Those suggested messages are great ones! However, to most Christians, in the end, whether you wear a seat belt or do self breast exams or whatever is not the end message the world may need. It would be something like "are you right with God?" and that would be offensive to most, I think. For many Christians, the most important thing any(every) person can do while here is get their life straight with the Creator. And the message "God loves You" is far less offensive than asking that question.
I don't think it's a stretch at all to think that a message about Jesus or God loving you could hurt someone who is questioning religion/who is angry at God/who thinks that God is punishing them. My mother had breast cancer a few years ago and she struggled with religion (well she was already struggling for other reasons with the Church) because she couldn't help but feel like God was punishing her because she'd stopped going to church. Now luckily she was cancer free after a double mastectomy--but what if she hadn't been so lucky? Maybe she would have made peace with God or just stopped believing altogether. But maybe she would have stayed in the place she was in--feeling like God did this horrible thing to her. I can't see how a person struggling with that could be helped by seeing a message that God loves them--I would think that would just make them scream, "Then why did He give me cancer!?"
I also don't understand why folks are saying that saying "Jesus" in the message might be not inclusive enough and hence offensive, but saying "God" is inclusive and hence not offensive

. So by changing "Jesus" to "God" it seems that Muslims and Jews can be included. But what about everybody else? Obviously atheists, agnostics, unitarian universalists, people who are "spiritual" but don't believe in a higher being can't be included if we take "God" literally. I don't know how Buddhists or Hindus or Wiccans or Pagans take the word "God," but from the very sketchy knowledge I have of those religions, the it seems the literal meaning of "God" does not really seem to capture what practitioners of those religions believe in. (For instance, since "God" is singular, how could it possibly capture what a polytheist believes in? To include these views one would have to say "The Diety(ies) love you.")
I teach a philosophy course where we look at arguments about the existence of God (these arguments all come from medieval Christian philosophers, so they really are only aimed at God as understood in Christianity/Islam/Judaism--hence we use the term "God"). I always try to be very precise about the wording I'm using because generally "God" is understood to refer to a being who is all-powerful, all-good, all-knowing, and is the ultimate creator and sustainer of the universe. Sometimes we are talking more generally and don't mean to only pick out the being described by Christianity/Islam/Judaism and then I usually say higher power or higher being--by which I mean just some supernatural being or force (which probably still leaves some conceptions of spirituality--I'm not sure it captures, for instance, the view that "God is love"--though really I'm not sure I actually understand what that views means anyway

).
If the skywriter really meant "God" to include all possible conceptions of a higher power, then the claim that "God loves you" won't make sense for some conceptions and just utterly false for some higher powers. The ancient Greek gods, for instance, were plainly not loving and never claimed to be. Those who are deists believe in a higher being who doesn't get involved in human affairs and may even be uninterested in humans and human affairs, in which case its hard to make sense of the claim that "God loves us." I think if the great diversity in religious/spiritual beliefs is acknowledged, then it's hard to see how even the most general message about a higher being/power could actually be inclusive.
In any case, I find it very hard to believe that a person who spends a lot of time and money writing messages about God or Jesus in the sky does so without any thoughts toward converting others or preaching to them--that he is just interested in brightening up the days of people who already believe. I don't buy it. I wouldn't buy it if it were an atheist who wrote "There is no God" either. And that's the part that bothers me. (That's not to say it ruins my day or anything. I see the message--I think it's a conversion effort. I roll my eyes. We move onto the next ride.)