T-Mobile Wins Judgment Against Pre-Paid Traffickers

Consumers are harmed and may be misled about the source and origin of their mobile phones, and they are sold phones without their manufacturer's warranties, accessories or user manuals. Because the phones may still carry T-Mobile's brand, consumers may believe they are purchasing handsets manufactured for T-Mobile and covered by original warranties.

So basically T-Mobile won the judgement not because their phones were being purchased and resold, but because their brand was still being associated with the resold phones.

It appears to be a case of harming the brand name, which is pretty much what I figured it had to be since the buying and reselling is perfectly legal if done properly.