I'd agree it can be annoying, and even costly, but I don't think a restriction of not allowing people to leave during combat would achieve much. Generally people leaving at those moments are doing so on some type of 'principle'. You're pulling too slow, the healer has a fishing pole equipped, etc. etc. and it would be just as productive for them to stand there doing nothing and either wiping the group or just making it a tougher pull. Unlikely they'd think "Oh, I guess they really need me" and jump into combat. They'd just wait until everyone died, feeling justified in their ability to kill the group, and then leave.

There are also situations where being able to leave while in combat wouldn't be the 'wrong' thing to do. For example if someone decided to pull the instance in hopes of wiping everyone.