Unfortunately, we just didn't design it that way, sorry.
In our thinking a channel is just an organization method under a Group. You can think of it more as a category tied to a specific group. It doesn't have permissions, it just inherits permissions from the group it is a part of.
The slight deviation in our original thinking is the "companywide channels" as that was a hold over from Stream preview.
We are currently reevaluating our thinking on "companywide channels." Our thinking is still a work in progress and "TBD", but right now we are thinking about changing "companywide channels." The idea we are toying with is make them be more along the lines of a special "stream only" group. If we ended up doing that, we'd be able to have a cleaner model (all channels are in groups), we could have permissions on them (because they are just a special group type), and we could allow you to have further categorization in them (channels within this special group).
Do you have thoughts on our idea to change "companywide channels" to be a "special stream only group"?
If you still feel like channels need to be able to be associated with more than one group feel free to add it as an idea in the idea forum for others to comment/vote on.