Who made you judge and jury as to what cell phone activity is acceptable and what isn't?
The point is, you don't know why they are on their phones, how long they have been,who they are, etc.
And because you don't, coming to judgments about what they should or shouldn't be doing is completely irrational.
That's NOT "noticing".
Is the lady next to you that is "thumbing" franticly in the middle of hall of presidents responding to a facebook message from a family member in another country? Or just updating her status? YOU HAVE NO IDEA! If you do know (and are not just assuming), you are paying wayyyyy too much attention.
Alot of people who visit other countries (even Canada) to not activate their phones for cellular use, but when on wi-fi, can access facebook and texting apps to communicate with friends or family members (possibly someone just on the other side of the park).
The fact that you, or anyone else here feels they can set a standard as to what is an acceptable amount of time and or topic matter to discuss electronically is just way off base.
Whether you admit it or not, talking about something negatively or saying others need to stop (as the OP says) is more than noticing, it's judging. Often times, prematurely without the appropriate information to do so.