The term I would use for such behaviour is voting :)
Voting imho is a subset of moderation.
The only restriction on voting is that it must not be personal, i.e. targeted at a user.
Downvoting multiple answers to the same question is entirely reasonable. If we think an answer is bad, wrong, misleading, not useful, is not an answer, etc, we should downvote it.
Here are some possible reasons why those answers might have been downvoted:
caioquirino's (now deleted) answer shows how things behave differently if we create an actual directory called ~, without explanation. This is actually quite a nice illuminating example imho and I don't think this post should have been deleted, but lacking explanation is a possible reason for downvoting.
d a i s y's (now deleted) answer, although it adds some interesting (if basic) material, does not directly answer the question, which is about why '~' does not expand and why we can't cd to it.
Your answer shows a way of investigating what's going on, which is helpful, but again is rather basic (appropriate to the question in that regard) and introduces extraneous and potentially confusing material (backticks for command substitution).
karel's answer explains the problem very directly, (and I think short answers like this are valuable because sometimes you just want a TL;DR so you can get on with your life) but the other answers explain the same thing in more detail, so this answer might be seen by a voter as unnecessary given the existence of more comprehensive ones.