I'm just wondering, why can't i post in the Trash Can section?

Well beacuse the trash can section is ment for unneeded
or inappropriate threads, not for posting

All threads in the the trashcan are automatically locked/closed.

why would you even need to post in there anyways?
It's only for threads that don't need to be around anymore

Trash Can is the place where threads go when they are no longer relevant and have no reason to be posted in. But we move them there instead of deleting them in the case that they need to be referenced back to for whatever reason. So just in case someone ever wonders the same thing as you just did, they'll find this thread to reference to in the Trash Can as well. :)