Why is “unacceptable” grammar accepted? It is accepted because if an Ashkenazi goes through the frum world talking about Shabbatot, talitot, etc., people will think that he is weird, or worse: that he is a Tzioni, ha ha.
One could write a book about all of the mangling of pronunciation and grammar (English as well as Hebrew) that are part of the normative frum dialect, but it would not change the way that people speak. As an example, take the expression “shala shudis”. Shala is not a word, nor is shudis, but everyone knows what shala shudis means, and hardly anyone thinks twice to say “shalos seudos”, or more precisely, “seudah shelishees.”
Perhaps there is some measure of pride and identity that come from not speaking in a fancy way; for example, to show that we are not like Maskilim who taught vocabulary and dikduk for the sake of their literature and theater, but neglected (or outright rejected) teaching Torah u’mitzvos.
Personally, I “am mispallel” to “be zocheh” to learn the intricacies of the Torah and Loshon Hakodesh. 🙂