I am not a lawyer either, but this sounds illegal to me.
I'm probably way off base, but the term 'restraint of trade' comes to mind.

I think the real test will come when ebay tries to penalize someone for avoiding paypal.

The obvious workaround to avoid paypal is to use the 'pay on pickup' option, and then the two parties work out the payment and shipping details via email. A lot of extra effort, and a lot of extra risk. Ebay is inadvertently encouraging the risky transaction/buying methods that they say they are trying to avoid. Go figure!