Quoting your post which links to this one:

Why should they not be free to do so?
They are free to do so, at the risk of alienating helpers who view the practice as incredibly rude.

Note that we're talking about the practice of copy-pasting the same question, usually a trivial one, to three or more different websites all at once and then cherry picking the "best" answer.

This differs from the less objectionable practice of asking a question once, and then using another website later as a way of getting second opinions or further help on the answers received.

If you answer the same question repeatedly in different places, that is your choice, and if you regret it, it is absurd to blame the questioner.
If you answer a question, then get ignored because the questioner got an answer on another forum and didn't bother reading yours, would you feel like your time was wasted? I certainly do, and that result is extremely common with shotgun posters.