The problem I have with this is that I try to do as much auto-detection up front as I can, which is why you see the ajax checks while entering the Space Name and Page Name fields. If using a parameter value in that field, the ajax check will fail, and the job itself will fail if you enter something invalid when initially building the job.
That said, if there's an error, I agree it's not my problem and caused by the person configuring the job. The goal was just to be helpful up front while configuring it.
As a last resort, I can just replace the current error with a warning if it detects a "$" in the space or page name. That just means it'll have to be up to you to make sure the parameter is formatted properly when kicking off a build.