It's not a monopoly. Do you know how many other tablets are coming out this year? Every company under the sun is releasing one. Steve Jobs preventing you from buying those? Nope. Do you have to buy an iPad to get on the internet? Nope. Is it a computer? Nope. It's a media appliance that can replicate a lot of computer functionality.
My PS3 can play movies, play games, show pictures, play music, browse the web, and run Linux. It also has dedicated processors preventing me from running any non-approved code. So if the iPad is a computer then my PS3 is a computer as well and by your definition Sony should open it up. But they don't and they won't. And that's perfectly legal.
Wow, a lot of replies. Since I'm typing on my iPhone I'll reply to two posts and try to answer all replies.
It is irrelevant how many other tablets come out. If the iPhone OS dominates the field, it will become a monopoly like MS was or still us if you talk to the EU. If you've paid for a movie or downloaded a music track before iTunes went DRM-free that means you are locked in. Sure you can either upgrade your music (for a fee) or use the analog loophole but most consumers don't know about that which makes it irrelevant. They average user may not even know tat current tracks are DRM-free.
Video games and e-readers keep being mentioned. This is not the same thing. The iPad is meant as a computer. Look at what is happening with Google vs China. China is censoring and taking authoritative control over, not really the web, but web content access.
Apple has been doing the same thing. Apple has recently told a well-respected European mag that they had to change their content because they viewed it as explicit. Check out the story by Jesus Diaz at Gizmodo. This is why it is dangerous for any one company to control the filter of content. Right now other companies don't have a chance. Apple is the most respected brand out there. When someone spends their money, they will trust Apple first because of reputation and experience with their products.