Originally Posted by
flyerbaby19
Is it against the rules for a passenger to do that? I was always under the impression it was first come first served for any open bin space (other than F)
"Rules." Often, that's an ambiguous term itself.
But yes, I think you're right with one exception - UA started marking the bins for E+ on many of their aircraft. So I suppose it's also against the "rules" to use one of those bins unless you're seated in E+.