occupied foreclosure, why did the bank ot evict?

[Lynnette E.]

I have been looking at some foreclosure properties and some of them are occupied.  

Why would a bank foreclosed property not evict folks that are in the property prior to selling it if they are not renters with a lease?  

If they are renters with a lease, it would have to be a long lease for the lease to still be active after the foreclosure process is complete.  It seems that the bank would get a lot more money delivering the property empty.

What kind of possible headaches would it be buying  a house with someone in it?  I am assuming it is a past owner who does not want to leave.