The employee makes 100 dollars in tips that week. This brings his total compensation to 250 for the week. The employer is required by law to pay the employee an additional 50 dollars - the difference between his current wage and tips, and the federally mandated minimum wage. Tip or untipped, he cannot have an employee that makes under the federal minimum wage (barring exceptions we won't get into here).
Now if we just got rid of a tipped position minimum wage, it doesn't matter. The employer would have to still pay 200 dollars to the employee, since he cannot make below FMW for that week. The tipped employee MW is completely useless and redundant.
I think it's important, though, that we don't think waiting tables is a minimum wage job...because it's not.