It's true that Obama has declared his willingness to negotiate with various dictators. But there are some key differences. First, Obama has not said he'd negotiate with terrorist groups -- indeed, the centerpiece of his foreign policy is destroying al Qaeda, not negotiating with it. I think there's a pretty clear principle operating here. When you merely have strong differences with an adversary, like the Soviet Union, you can negotiate. When your adversary is committed to your destruction, there's nothing to talk about.
Now, it's true that Hamas is not committed to the destruction of the United States. But it is committed to Israel's destruction. And Israel is our ally. Now, you might think that makes the principle of not talking to enemies would be weaker, but it actually makes it stronger. We can negotiate with our own enemies at our own risk. To negotiate with the enemy of an ally is to undercut your ally.
Matt weighs in:
I think it's perfectly reasonable for an American president to say that he wouldn't have any diplomatic talks with Hamas as long as that's Israel's position as well -- after all, what would they talk about? Hamas can't make concessions to the United States nor is there much of anything the United States would concede to Hamas. So in that sense, Barack Obama's refusal to expand his generous meetings policy to Hamas is both defensible policy and a good cheap talk way of saying something that "pro-Israel" folks like.
The more meaningful question facing an American administration would be what kind of counsel/pressure/whatever they give to the government of Israel regarding holding talks with Hamas.