On a classic arcade forum I also frequent, they do this for a token design every year or so. The nice thing about doing it this way is once the entries are in, they hold a poll to see which one people like most. Afterall who's going to be buying these things? Probably mostly members here.
On the other hand, if Steve wants to retain copyright and/or certain logos associated with this site specifically, then it's better to pony up the cash and sell it to the masses. He retains more control over the site and it's trademarks this way.
I think the deciding factor is whether the shirts specifically promote www.pizzamaking.com
or not. Without doing so, this exercise is kind of a moot point. I think everyone WANTS to promote the site to show they belong to the 1% of pizzamaking.