Because of the permissive MIT license, they can freely incorporate each others codebase, as can any software project, commercial or not, open or closed.
The Linux codebase, on the other hand, is licensed under a copyleft license that only allows its use in open-source projects that themselves only allow their codebase to be used in open-source projects, and so on. Because of this, Linux can incorporate BSD codebases, but not the other way around.
The Linux codebase, on the other hand, is licensed under a copyleft license that only allows its use in open-source projects that themselves only allow their codebase to be used in open-source projects, and so on. Because of this, Linux can incorporate BSD codebases, but not the other way around.