They didn't "switch it off" they merely turned off form submission blocking when the form was invalid. It's back on in chrome now because they have the error notification bubble. The constraint validition API, the error pseudo-classes and the extra attributes all work in Safari.
If the answer to "are the contents of this form valid" is always "yes", I don't really care how much of the validation API the browser supports; it's effectively going to waste.
I'm glad to hear it's turned back on; I hope the form-validation interface is as sleek and professional-looking as the one in Firefox 4.
