BGP advertisements of peering points

Forgive a newbie question from a new PAIX peer...

I've noticed some ISPs (BBN (*ahem* GTE), UUNET and Verio specifically)
are advertising the PAIX peering network (198.32.176.0) as originating
from their AS.

Is this a common practice for all ISPs, or just enough to make the IX
reachable?

-Andy