I'm not sure if this is a valiant attempt to confuse Americans, but this isn't true. Trains stop in both directions.
A reason it might not seem like that is that there are two branches of the Northern line running in parallel between Euston and Camden Town, and Mornington Crescent is only on one of them. So depending on which way you turn when you reach the southbound platform at Camden, you may or may not pass through Mornington Crescent on your way into town.
Nowadays, the next station (Camden Town) and that area's various alternative/subculture markets make the area very busy all the time, so all trains stop at Mornington Crescent at all times.
Train nerds will doubtless be fascinated that geographically it's on the east of the other branch, but on tube maps it's shown on the line to the west.
There are some complicated track crossings in the area and it's easier - and more useful - to keep the branches straight on the tube map than to show what's really happening underground.