That's why Ashe is alright. And only Ashe.
The problem is that Rylai's offers a significant amount of HP, and if it were to be even closer to balanced, it would offer more HP and less AP. Rylai's, much like Frozen Mallet, enables the 'high durability champion you generally can't fight' aspect on mages at the moment, especially with the range, armor/mr, and actives/passives that common itemization provides for them.
There's no reason Frozen Mallet as an item can be considered balanced if Skarner with permaslow on Q can't. In the same basket we have Rylai's, which enables similar gameplay but generally at a range and with more CC/burst inherent on most of the users.