Depending on the contract Lewin had with MIT, taking down the lectures may have been the only option MIT had. At my previous institution, the faculty contract was very clear that if a faculty member left the institution, the university had no rights to use materials the faculty member had created, aside from items retained as records required for purposes such as accreditation reviews (e.g., course syllabi—and even then they couldn’t be reused, they were just kept on file). If Lewin was working under such a contract, once Lewin was no longer affiliated with MIT, MIT would have had to take the videos down.
No idea if that’s the case here, but clearly Lewin has retained his rights to everything he created, and has been posting it all on other venues.