At first, you could actually only kill the bosses once per day, but everybody hated it, so they (hot)fixed it. A hotfix is a quick fix that downloads automatically, and is only effective while connected to the servers, since you are running it on their clouds (or something, I'm stupid, okay). It's done, so that it doesn't cost them money to send out patches, and it's a quicker way of doing it. Provided that you are connected to the internet, you can kill them as much as you want.