Unethical because you are presenting yourself under false pretenses, taking interview slots from people who don't have jobs yet, and are possibly breaking the rules of your placement office.

It will come back to bite you in may ways. If your current employer finds out what you're doing, they may go back on your offer. (How would you like it if they kept interviewing people after they offered you the job?) If Google or Amazon find out, you may get blacklisted in the future. Classmates who see what you're up to will not trust you in the future.

Net - protect your integrity, as it takes years to make a reputation, and minutes to break it.

EDIT - fixed spelling, and add one more point - If you want to work at one of them in the future, get to know the HR reps, and keep contact with your classmates that are going there. Let them know what you're up to. They're the ones most likely to pull you through in the future. (And block it if they see you behaving poorly in the meantime)