Most places in the US require businesses to "make up" the delta between minimum wage and what the employee receives through wage+tips. Seattle just codified this more clearly by having a "minimum wage" and a separate (higher) "minimum compensation". So while wait staff have a minimum compensation of $15/hr, that number is inclusive of tips they receive. Their minimum wage (what the business has to pay regardless of tip) is lower at $12/hr.
People may have considered no longer tipping because of the publicity behind the new-to-them-but-not-actually-new concept of minimum compensation but it's effectively the same system at a higher number.