It's not really a subsidy though, it's just a complication of the payment.
It could be a subsidy if it wasn't an established custom to tip, but people know quite well that they are paying the server a tip for the service.
I suppose the fact that it enables the restaurant to under-report wages and thus under-pay wage taxes is a bit of a subsidy (but then some of that savings probably shows up for the customer).
It is a subsidy, because the server gets the same, it's the restaurant that pockets the difference.
Say the server works 40h/week like a regular full-time worker, so the minimum take-home pay under federal law is $15000. If the server gets $0 in tips, the restaurant must pay the $15000. But if the server gets, say, $5000 in tips, the restaurant can pay only $10000.
The server gets the same money, only the restaurant gets to save some.
It could be a subsidy if it wasn't an established custom to tip, but people know quite well that they are paying the server a tip for the service.
I suppose the fact that it enables the restaurant to under-report wages and thus under-pay wage taxes is a bit of a subsidy (but then some of that savings probably shows up for the customer).