Another point, and something I brought up to @duketheslaya in that other thread, you don't really know if someone's doing something wrong or if you're missing relevant information like their injury history, goals, overall programming, etc.
Do some (many?) people use crap form? Sure, but not all people all the time. Seeing a few reps from across the gym definitely doesn't give you enough info to go in and say "Stop doing that. Do this instead."
You see someone doing this exercise, do you go up and say "You're going to destroy your spine."?
You see this, do you laugh at "the dork doing useless half reps"?
Bottomline: When it comes to giving advice to strangers, just mind your own business.