It's not exactly an "evil plot" but it isn't totally innocent either. The very employees at Microsoft who decided to do this probably cannot themselves accept a gift this size from a vendor... or a would-be vendor.
Almost every big company has policies like this. If you're in advertising or promotion you're supposed to know that you just don't do this. It puts the recipients in an embarrassing spot.
Microsoft's advertising people know this. They thought about it, and then they did it anyway. I think that tells us something about Microsoft that we would just as soon not know.
Having never worked for the Feds, I'm not sure what their laws are, but at my level, it's not illegal for them to offer the gift, just for me to take it. On the other hand, I frequently receive evaluation copies of textbooks, and see no conflict of interest, since I have a legitimate role in evaluating these books for classroom use.