Multiple Contextual ad networks on same page

I have seen many websites and blogs specially social networking and file sharing sites have adsense, adbrite, YPN and bidvertiser on same page at different locations i.e. adbrite on top and adsense in sidebar and even sometimes bidvertiser on bottom! Is this violation of TOS of contextual advertising or not

 

I believe it's violation, but those sites can be premium publishers so they have special conditions.

 

I am not sure even premium publishers are allowed both adsense and ypn on the same page.

 

Yeah, lots of sites have every known advertising venue on their sites.
Some people never read the TOS of the advertising they put up.
So boom, they put up adsense, ypn, adbrite, etc all on one page.
At some point, their accounts will be disabled or banned for doing so.

Got to remember, adsense for one checks before they pay out.
They see a site with all this, they don't pay.