Why did he throw that?

[Finwe]

Actually, I don't think that he would have had to be "dumb" to not know what the palantir is. Only the Wise knew of the palantiri's very existence, and even fewer of them knew that some had survived.

As many people have said, Grima probably threw the palantir out of a desire for blind revenge. In his heart of hearts, he knew what he was doing was wrong, and that listening to Saruman wasn't going to get him anywhere good. In fact, I rather think his love for Eowyn (no matter how twisted it was) helped to keep his deep-seated love of Rohan alive. When he saw Saruman being overthrown, perhaps he thought that he could somehow "contribute" to Gandalf's cause, and give him the palantir. I'm sure that he knew what it was, and what it did, after watching Saruman. I know it seems like that action is too good for Grima, but I truly pity him, and I truly believe that there is still some good left in him.