[TheLoneWolfling]

If it's specified, I don't mind people relying on it. That being said, Java is over-specified in many ways, this potentially being one of them.

I know Python does not have this guarantee.

Personally? There should really be a couple different RNGs in Java's standard library - all combinations that make sense of [insecure/secure, can set seed / can't set seed, thread-local/global] (local/global being without distinction if one cannot set the seed). The default being secure / can't set seed.