Presumably there’s a more formal name for this. If some people, some part of a class of them, do this then to assume that all do is a fallacy.
That welfare queen turning up in a Cadillac to collect her checks doesn’t mean all on welfare are shamming it – only that one has done so. That one adult make became happier when spayed does not mean that all teenage girls who – currently – desire their tits flayed off will be forever happier.
So, what is that more formal name for the fallacy? Assuming that because one then all? Fallacy of generalisation?