I've also been thinking about WWE's stance on sponsorship, in that it is strictly forbidden for superstars to wear sponsor branded clothes or promoting non-internal products (WWE superstars are sponsored by companies such as Mattel by default). I think it's a foregone conclusion that companies reach out to WWE superstars and offer them freebies if they'll use their products in public - it's just that WWE won't allow this practise on TV, which sucks because a lot of lesser earning superstars could do with that kind of financial boost.
Brock Lesnar is obviously an exception, in that he is allowed to display his sponsors on TV, since it's part of his gimmick. I also think it goes deeper than that, in that it's quite possible that Brock still has contractual obligations with his sponsors and/or that he negotiated the right to wear his sponsor gear during contract negotiations earlier this year. It seems that Shawn Michaels is sponsored by Carbon Express, as evidence by his wearing of clearly branded Carbon Express clothing in most of his public appearances since retiring.
If WWE allowed it's superstars to receive external sponsorships, it could easily partner with those sponsors to supply merchandise of the superstars in exchange for a healthy cut of the profits.