Not necessarily. For example, there is no question that the massage rooms in the fitness center are not intended for guests to give their spouses a massage. That room is intended for a certain purpose, the delivery of a service for a fee. The resort may or may not keep that room locked, or otherwise prevent guests from using that facility of the resort. Guests cannot assert that they must have access because they paid their room rate. Other examples include function rooms (such as Ariel's at BWI), cabanas (at CR), and business center equipment. Some of these things are things that guests can use, even though they're not intended to be used in that manner, because the resort turns a blind eye, while other things the resort doesn't turn a blind to are often locked down better.
I mean this good naturedly-- but wow, you seem intent on living up to your screen name, here. If a mother parent brought food or drinks for a child with allergies, or baby food for an infant, and gave them to out to their family, would you be bickering about this? I really don't think so. They sell bottled water at the food court, too...are you going to scold all of us who fill up our own water bottles in our room before we leave it?
They are buying food from the food court.
It's not as if they are bringing their baloney sandwiches in and using it as a picnic facility.
If I bring my own tea bag to a restaurant and ask for a cup of hot water, am I doing something wrong? I do it all the time...not because I'm cheap, but because I have digestive issues and peppermint tea often helps me if I'm out and about and not feeling well. The OP doesn't drink caffeine and the food court only offers limited options, so they are bringing their own. Why does that bother you, truly? Why should they have to go through the machinations of bringing mugs from their last trip (and possibly be accused of re-using them!) and pour soda into it? To appease people who have no business worrying about what another family is doing in the first place?
If there were signs posted that said 'no outside food or drink,' then the answer here is simple. Since there are no such signs (to my knowledge), each person is free to exercise their own good judgment.
Your analogies are NOT analogous. Can you use the massage room to give your spouse a massage? Of course not. Can you use the massage room and bring your own brand of massage oil for the masseuse to use? ABSOLUTELY. This is not the same thing as guests of one resort pool hopping or using Mugs from other resorts at other places in the World, etc. Using that line of thought- you also shouldn't be able to bring your own drinks into the parks...I mean, hey, they sell beverages there for a fee, right? Would you tell the OP they shouldn't sit down with their own drink in Cosmic Ray's if they bought a burger and fries but had their own bottle of water or can of soda?
OP- bring your own cans. And enjoy them. You're paying a pretty penny to stay at ASM, you're buying food from their over-priced food court, you go right ahead and don't let the naysayers on this thread convince you otherwise. The only people who will be agitated are likely to be the ones who wish they'd have thought of it!