They are just trying to discourage people from sending MSC because BTC is much more liquid and thus more valuable right now.
Then it would make no sense to accept MSC in the first place. But it could be that they received more MSC than expected, so market depth comes into play.
ya.ya.yo!
The reason Mastercoin foundation got safecoin abroad to do the IPO with MSC was because a lot of people have a lot of mastercoins but nowhere to spend them. So a lot of capital is/was stuck in mastercoins. So the reasoning was that many of the investors would jump at the chance to invest possibly a lot of MSC if given the oppurtunity and so safecoin is the perfect chance to do just that.
Safecoin would have in no way got so much investment (including BTC) if it weren't for the MSC investors who started the train rolling with lightspeed.
Now this seems to be quite a good thing for mastercoin since at this moment about ~50,000 MSC have been put into safecoins hands and they sure as hell won't be selling that in the near term. Possibly piece by piece in the coming years. So a lot of MSC have been taken off the market and at the same time mastercoin has and will make a better name for itself. Now maybe more IPO's could be looking towards the mastercoin idea, since people here are willing to invest in good ideas. This will cause a positive loop effect.