Companies choose to be on Shark Tank. Nobody is forced to be on the show. The show is just a giant informercial; in primetime. It's definitely an informercial that I love to watch.
Here's a excerpt from the American Idol Wikipedia page, talking about the contract:
"The contract the contestants are required to sign gives Simon Fuller's company the right to oversee not just the recording deal for "American Idol" stars, but also control any merchandising, touring, sponsorship and movie deals."