We do forbid it, though: https://en.wikipedia.org/wiki/List_of_anti-discrimination_ac...

It seems that we have decided as a society that we gain more from the cumulative effect of all people gaining a equal and fair chance, than we would save by allowing people to use whatever branch predictors they like.

It looks like branch predictors in general tend to have unforeseen problems; see also https://en.wikipedia.org/wiki/Spectre_(security_vulnerabilit...