What’s With The “Should Be Previously Unpublished”

I’ve just started submitting to poetry and fiction magazines online. Some of them are very keen that a particular poem shouldn’t have been published anywhere online, including on author’s own blogs and writing social media accounts. I think that is particularly bonkers, because it doesn’t let a poet/writer to be able to even think of freely sharing a poem without the at least scintilla of feeling —  what if I submit this poem in the future? I think it’s a disgrace on free speech. I know that it must be tiring to check submissions for plagiarism, but imagine a code of honour, like in the universities I’ve heard (Carnegie Mellon, for example, through a friend who studied there), that if a person is submitting, and he wishes to keep the original copy of the poem published on their blog/social accounts, they should be able to themselves tell this to the poetry magazine they are submitting to. Wouldn’t that be a win win situation?

Of course I know jack shit about copyrights and legalities. If you have any idea how this idea would uphold the legal things, comment?