It may vary by state, but my understanding is that employers are required by law to pay *every* tipped employee the standard minimum wage if their tips don't get them to that level. So normally the tipped worker gets the tipped worker minimum, as low as $2.37 in a lot of place. If their tips don't bring them up to the regular minimum wage, the employer is supposed to make up the difference.
In many places this allows tipped workers to make well above minimum wage. But at a place like Sonic where not a lot of big tipping is taking place, it might be just subsidizing the employer as Morledzep says, allowing them to chip in less. Cash tips are different, but only if the employee doesn't report them.
And on top of that, if the Sonic actually pays servers regular minimum wage, they can take ALL of the tips and distribute them to all the workers, even the non-server employees such as cooks. That might sound good, but it is really just a way for a restaurant to avoid giving actual raises from their own pockets. (Some states have stricter rules.)