It's rent-seeking if, instead of selling you the font once under a mutually agreeable license, they charge you an ongoing fee to "use" the font under conditions that the foundry controls and can change without re-negotiating the contract. This is especially true if the condition are "you can use for purposes x, y, and z, but if you want to use it for a, b, or c you have to pay a higher rate".
What do you mean? If you agree to the contract when you rented access to the font, you're subject to all its conditions. If you don't want to pay the additional rent, your choice is to break the contract and lose access to the font, or pay up.
