I wouldn't call that unethical - even free users would benefit from such a system, in the same way that non-users would also benefit, but paying users would benefit even more.

The system actually requires itself to be ethical. The ability to make things better for the paying users is dependant on the system having a lot of free users. Those free users have to be getting some utility out of the site, otherwise they won't use it. And without large numbers of free users, the paying services can't be offered!