Why force the bill through in its rawest, most extreme and most batshit form?
Two possibilities. One they really believe in this and therefore they want the law to be as extreme and batshit as possible to match their genuinely-held extreme and batshit views.
Two, they think that by making it really extreme, someone else (eg Westminster, as you suggest) will intervene, so they will have the relief of their crazy law not being passed while simultaneously being able to blame someone else.
It's really hard to tell. The first possibility is frightening, the second one is highly risky. Both are mad in their own way. As Bernard says this could be the beginning of the end for the SNP.