Ezra's certainly right to say that it's bizarre for George W. Bush to criticize Barack Obama on the grounds that "it'll send the wrong message" for Obama to hold a meeting with "a tyrant who puts his people in prison because of their political beliefs" considering that Bush does exactly that on a regular basis. Is it a good thing that the people of China and Russia and Saudi Arabia are, like the people of Cuba and Syria and Iran, ruled by dictators? Of course not. And if the lessons of history indicated that some kind of "no meetings ever" policy caused those regimes to melt and transform into wholesome democracies, then we wouldn't be having this debate.
But things don't work like that, and in the world as it is it's hardly practical to eschew all meetings with everyone whose political system you don't approve on. The question is, thus, whether or not this posture of creating a mostly arbitrary class of "bad guy" that we're going to take down with our awesome powers of snubbing accomplishes anything meaningful. Obama's contention is "no." Bush's contention is "yes" but he has absolutely nothing to show for it.
Meanwhile, I think being attacked by the president helps Obama. These kind of criticisms may carry some weight when delivered by Hillary Clinton or John McCain, but having Bush give voice to the same concerns merely underscores the extent to which Clinton and McCain are arguing from a position that's deeply continuous with the mindset of the Bush years. How many people are surveying the mess Bush has made of things and thinking "we need more of the same?"