Hackhands.com deducts from tips?

Hackhands keeps 20% including 20% of your tips.

Hackhands gets around gratuity laws (such as the one linked below from California) by stating that their experts are not employees and that they are simply connecting user and expert. If that were truly the case, it would make more sense to pay a flat commission for the connection.

By paying a percentage of gratuity it seems that Hackhands is asking for compensation based on the quality of the content and service the expert is providing. This seems like a potentially problematic behavior.

Can anyone explain to me why they should deduct from gratuity instead of just having a fixed fee?

http://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=LAB&sectionNum=351.

"What is the HackHands service fee? Our service fee is 20%. This amount is deducted from the total amount charged/earned including tips. This service fee goes to our operations, enables the infrastructure for our platform."

https://hackhands.com/create-account