That's not what this is. Keeping interviewing after you accept a position is very much unethical. Switch it around, companies would be unethical if they kept interviewing candidates after one had accepted an offer. Can you imagine how you would feel if 3 days after accepting an offer, the company called you and said they are rescinding the offer because they found someone better?

I don't think most of the commenters here have spent much time hiring. Usually, once someone accepts you go tell all other potential candidate the position has been filled, thanks for apply, keep your resume on file, standard boiler plate. The whole hiring process is shut down almost immediately -- largely because hiring is usually done because your team is over worked and the whole hiring process adds to that work. Now it has to be restarted and new candidates found. I've had this happen to me a few times, it's often like starting from scratch again. Essentially the person who does this screwing primarily people like themselves.

Of course, regularly in our industry, get multiple offers -- leveraging them is totally fair competition. Or getting offers, after you accepted, from companies they previously interviewed with in your search is totally legit -- just tell the orignal company ASAP. But to accept and keep interviewing is some scummy shit that only hurts your fellow engineers.