Isn't that against MP rules to begin with ?

I reckon they should honour it. Unlike Lufthansa, UA did this for me and a friend of mine. Even though my situation was different as my MP number came up as 'nonexistent' when flying ex. ZRH (the early ZRH service, not today's) so I was given a temporary MP account which I had to merge. They did not really believe me but were helpful nonetheless.