The United CRJ-700 are fitted with 6 first and 60 coach seats. A row of seats taken out for economy plus. United's EMB-170's are fitted with 6 first and 64 coach seats. Again, a row of seats are taken out for economy plus. So I believe United is limited to 70, they just choose to fly the CRJ-700's with 66 for premium seats.

Thanks for the info about American. That is good to know.