Because they are explicitly inviting him to come hang out and work on anything that is of interest to him?

It's like two CS guys who both have an interest in solving big problems efficiently. They have ideas. They talk to each other about their ideas. They really love working on their ideas. One of them gets a ton of research money so he asks the other to come work with him in the new lab allowing him to work on his ideas as he wants.

Because they've probably discussed the possibilities beforehand...