To be fair, I think that most of us are just tired of being bullied and threatened (which does happen regularly here and in real life) because some people feel that they don't want to pay for a seat but the fact that they have children trumps every other situation.
Just imagine how uncomfortable a situation it would make for someone like honugirl - she has a definite and definable need, chooses the seat which meets her need, but someone assumes by her appearance that she can (and should) move. It happens all the time. She shouldn't have to explain herself or her needs, and nor should anyone else. (Sorry to single you out honugirl,
but it really irks me to see people make assumptions or think that their need trumps someone else's needs. You have a need, and hopefully you won't ever find yourself in such a situation when you fly)
There are tactics which one can use to
ask passengers to move to another seat, including what others have listed. Threatening and bullying won't work as effectively as asking or offering some form of incentive, be it a better seat, a beverage, or just a polite request.
But I agree - there is no law requiring airlines in America to force passengers to move to seat families together. The best option is to be proactive and pay the seat selection fee, or early boarding fee. If you don't want to do that, assume that there is a risk that you won't have seats together, and prepare a strategy to request assistance from others.
Happy flying!