When I say "my feeling is...", that doesn't claim omniscience. It's pretty clear it's just my supposition. I don't know for sure why DMG does anything.
But since you asked, here's my speculation. Why did they buy the series? They thought they could make it a success. They tried to nail down their control by running out the manufacturers, who had, in DMG's eyes, too much power. They thought they'd make it like NASCAR. Trouble is, the manufacturers were the main source of money. So it didn't work. The sport then went into a downward spiral due to the economy, lousy demographics in the motorcycle industry and declining sportbike sales. The downward spiral is self-perpetuating as teams like Buell and Jordan leave. Then there's no TV package, fewer races, so there's no sponsors, and the spiral continues.
Consider this, too. There has never been any evidence presented that DMG paid the AMA a single dime for the series. AMA leadership refuses to say. The AMA's tax filings, which are public record, have not shown any income from such a sale. So maybe DMG "bought" it because it was free.
And I agree with you, the AMA is ultimately responsible. Especially if they "sold" it to DMG, which ran it into the ground, and then used an escape clause to avoid paying anything because the series was no longer worth anything. (Again, not omniscience, just speculation by others who are fairly well informed.)