Same reason people mined Bitcoins a year ago when it was unprofitable.
Not the same situation at all. That days you couldnt mine another XXCoin with less difficult/price ratio to buy BTC with.
Keep in mind that there are much less namecoins in existence than bitcoins and that will stay that way for some time plus namecoins have another usage that bitcoins currently do not. They can register domain names in a decentralized way. You own the domain and no one can take it away from you.
You can also register .bit domains with bitcoins because you can change them for namecoins. The point here is that with the same amount of mhash/s you can get more namecoins by mining bitcoins and trading them for namecoins than mining namecoins.