The most charitable explanation I can think of is that this is a relic of old times, and that is changing. I know of founders who have insisted on this being paid by the VC, and successfully gotten so.

But I'd also like to know how standard/non-standard this is.. and why is it even the case in the first place?

Given how Fred Wilson (and others) talk about not liking SAFE, they really should then make it easier to do priced rounds without putting the burden on founders.