Rams are the only early siege units, and have a low power, so the AI must build many of them to reach it's quota. This is a bad strategy ... but only for this specific case. Normal catapults, trebuchets, etc are stronger and it is fine to build these, since they are offensive units.
Why can't we exclude them from this ratio check ?
I've noticed that siege units are also excluded from the 1UPT check. After thought about it, it may be finally for the best (a ram will be too vulnerable alone on a tile).
Do players think siege units should be counted in the tile limitation or not ?
The actual balance i've found for 1 unit-per-tile is 5 units on city (to be able to defend it). It's a good attack-defense ratio.