Playtesters rights?

I would say no, it does not, but then, your playtesters might not agree.

If you are worried about any of your playtesters might feel cheated if you adopt their ideas and not pay royalties or written credit in the rulebook or whatever, maybe you could start your playtesting sessions by warning the players that whatever they suggest you might or might not adopt, and that all suggestions are going to be considered as ideas they give you permission to use freely in your game.

In general, experience tells me that it is the opposite you should worry about: playtesters feeling hurt if you do NOT accept their suggestions, especially if you are too quick to dismiss them (oftentimes they will suggest things you already thought and dismissed because they didn't work, or simply because you didn't want to go in that direction, but remember that playtesters are trying to help you, and they want to feel you value their input).

I agree completely with seo.

The only testers I've ever had who asked for rights or royalties were close friends kidding. Most people are overjoyed with the knowledge that they helped make a better game.

I've had FAR more trouble with testers when I don't implement their ideas. People always see their own opinions as correct and sometimes react harshly. Just do your best not to offend anyone or make promises you don't intend to keep.

Finally, I'd recommend you reuse play-testers who do good work and avoid ones that cause trouble. While this won't make your current design any easier..., having a group you know and trust can help future designs. Playtesters will feel appreciated when you invite them back and you'll get a better design.

Well this is probably true if you do not present to them the facts about your game. For example, in my current game I am at the 6th prototype... When I playtest, I say that the game is in it's 6th version. By now we should be ironing out small details not changing mechanics.

But you need to explain to playtesters how mature a game is. This will give them a better understanding about the game.

And, an aside, if they suggest things that does not mean you can implement them within the framework of your current game (basically they just won't work). Your job is to convey how mature the product is (even if it is a prototype), such that playtesters know how much effort you have already put into the game itself.