Since I was curious, I did a little research.

I was incorrect in stating that the law requires a service charge (so much for depending on English-language news sites). The decree from the Superintendcia de Industria y Comercio only states that IF a restaurant charges a service fee, it must be made clear to the customer that it is voluntary.

The text of the decree makes it clear that the fee does not have to be 10% (the text says "XX%"), though other articles indicate that some people are under the assumption that it does.