I see this being challenged fairly soon, as Washington state's constitution prohibits rent control (RCW 35.21.830, for those interested).
It is certainly becoming a lot harder to be a landlord in Seattle; the current city administration is tying our hands on a number of issues. In addition to the pet rent/deposit change, we are no longer able to charge administrative fees, must credit the application fee to the deposit, and must offer up to six months to pay off the deposit(s).