To be clear, OP was mixing e-cig rules with CPB duty rules. The e-cig rules exist not because of their nicotine content, but because of their battery content. They must be carried in your carry-on. The CBP (and other country equivalent) rules are about bringing cigarettes into the country, not onto a plane.

In other words, you could literally fill your carry on bag to the brim with (analog) cigs and fly EWR-IAH or LHR-EWR, but in the latter case you have to pay duty for any amount you're over the duty free limit. In either case, UA doesn't care.