This is not a “made up” rule. It’s a rule of one of the UA Express carriers. (Commutair)
They’ve declared the E145 as a single item aircraft in their policies and filed as such with the FAA. They have no choice but to follow it.
Someone went through this recently and even got UA Executive customer support involved. They got to the bottom of it and the fact is, it’s legit, it’s being enforced and it’s not going to change anytime soon.