There's a big difference between buying a small vocal PA, and a big PA system for the whole band.
If it's only a small vocal PA you're talking about, I think it's only normal that the singer(s) provides it.
If you're talking about a complete PA system for the band, the whole band should pitch in.

Thing is though, you don't need a complete PA system for rehearsels, and hopefully when you play live a PA is provided by the venue. But, you DO need a vocal PA to practise.

That's just my 2 cents.