Why?
1.) At the lower level, individuals will have more difficulty serving. Poor tosses, balance issues, and other bad habits will lead to a greater propensity to footfault.
2.) Players lacking knowledge of the rules. You can't enforce something if you don't know whether it is a rule or not.
3.) Because of (1) and (2), you see a reluctance to call footfaults even among individuals who know the rules:
Firstly, some people simply won't call them if the opponent doesn't seem to gain an advantage.
Secondly, some people won't call them as they want to avoid potential conflict (either opponents who don't know the rules and get upset, opponents who don't think they are footfaulting, or opponents who think it is polite to avoid calling footfaults).
If I were playing a social match, I would probably point it out ('just to let you know, you might want to try working on keeping your foot off the line/court when serving', etc). In a competitive match, I would invoke the rules (warn, search for official, call or let it be called if I found one). I say probably, as I have never played an opponent where I noticed a footfault. My return is one of the cornerstones of my game, and letting my attention wander to my opponents feet instead of actively tracking the ball would not be helpful. Maybe in doubles, but I play so little doubles, that I can't remember ever seeing it happen.
I don't think it's acceptable to let it go *assuming* you see it, but actively watching for it doesn't seem reasonable for certain levels of play.