indeed.
And I believe not only shirts...
I also believe they alraedy thought about that (can't imagine they didn't) but I think they decided to wait with this kind of income until BB has more users (prizes per shirt or other stuff gets cheaper if they can order at large, so they could make more profit) and also, and most important, they need to spend their investment money they currently have on other things that are essential to keep the game going at this moment.
Don't forget they will need to spend money on it first, and only after that can recover it and make profit...
I'm sure once they have some budget they can invest while the game is running smoothly, they will start thinking about some merchandising material.
The idea is very good though, I'm sure they will have such things in the future.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.