I have a couple that I would like to be able to watch on my tablet, but it plays only MP4 format for videos.
I'm assuming your "tablet" is not an iPad? iTunes videos are MPEG4, but they are "protected" to play on only authorized Apple devices. (Thus, it is not a simple format problem.) iTMS music is no longer "protected," but the content owners
still demand it for videos, so...
apple really are irritatingly propriatary in everything
Don't blame Apple. Other services (and content providers and manufacturers, etc.) have their proprietary aspects, too. Why don't Android apps work on iPhones, or Amazon videos on Apple TV? Why can't I put diesel fuel in a gasoline car? That's proprietary! It's unfair!