It is the rules, IIRC we had one chalked off for that reason earlier in the season. Funnily enough I think it was Lynchy as well!! I always liked the idea of the American football posts that Hunslet used a few years back, they didn't get in the way. The flatcappers of the time got them outlawed though.

Try scored at the front base of the posts is not allowed. Law 6, note 3.a

It is in union. Law 22.4b