In practice this is unlikely, but it’s still the legal requirement.
A tip, in the classical sense, is expected to be something that the person you're tipping gets to keep in addition to their wage.
A minimum wage isn't supposed to guarantee a minimum level of welfare (that's what direct-transfer welfare programs are for); it's supposed to guarantee a minimum proportion between labor-in and compensation-out.
In fact, think of the alternative case: it would piss most people off to realize that giving $20 to a person on welfare, means that—if the recipient is honest—the government will subtract that same amount from their welfare check for that month.
That's the way that system is supposed to work, and people still find it unjust. So why don't they find it unjust when it's a system (minimum wage) that's not supposed to keep people "at level", but rather to increase their compensation as they provide more labor?
One of my biggest hesitations with tipping in places where it is unexpected, like a muffin shop in the OP, is whether the tip goes straight to the employee whose performance I tipped for, split among all staff indiscriminately, or more sinister straight to the employer.
By that logic, tipping is not an expectation since the law says that if the tips do not equal enough to bring the worker's earning up to the minimum wage limit, then the employer has to pay them the actual minimum wage and not the tipping minimum wage.
Many employers will not follow the law, but the solution to that is to punish the employers and empower and incentive the employees to seek out enforcement of the law.
It's not a "tip". It's an expected (and the expectation is in law, as evidenced by the lower minimum wage) part of the employee's salary.
This is a really bad system, (as David MItchell would say, it's a tax on conscience), but it's the system the US has. It's very different from almost all of the rest of the world where a tip really is a tip intended to reward good service.