You can't blame SEGA for coming to this decision. They've tried harder than most 3rd-party's to bring "mature" content to the Wii. HotD:OK was good but short and The Conduit was an average game in an under-represented (on Wii) but popular genre. I get the feeling the art-style on Madworld probably put people off. And if I remember rightly High Voltage already have some titles coming for Wii so these probably won't be the last mature games on Wii from them, but the next batch could be...
At the end of the day it's the consumers that'll choose.