Where is tipping "mandatory" by law? Is Pennsylvania like this state-wide?
Taking up seats in a restaurant has nothing to do with the tipworthiness of the service.
This whole situation in North America is the fault of the wait staff for allowing the venue owners to refuse to pay them adequate wages, and instead letting the customer be the bad guy when they are dissatisfied with the service. If foodservice staff collectively demanded fair wages across the board from their employers, they could shut down restaurants who refused to pay them, and any tips given on top of that would be those where they were truly earned, like bonuses in the rest of the work world.
The catch is, if they were paid like wait staff in other parts of the world where service is a career, they would be obligated to actually give 100% in their jobs and provide the best possible service every time, to best claim a stake in the success of the business. It is rare that I meet wait staff in (especially) North America who yearn for this tier of professionalism.
Instead, we have entitlement. Knowing how to press the "C" button on the drink gun behind the bar when pouring me a Coke doesn't earn you an extra $1.00 each pour in my books. Skilled sushi / benihana chefs (who spend five years in Japan learning how to make rice)? Sure. Carrying drinks to a table. Nope.
This closest analogy I can imagine is to some kind of pyramid scheme or sales pitch or something. Fifty years ago, the employer tells the staff, "well, I could pay you a decent wage, but why don't I pay you a pittance, and you can demand the rest from the customers yourself once you earn it for providing the best possible service! You are going to provide that level of service, now, aren't you?". The service staff just gulps, and nods, and does whatever they feel like on a given day.
To my mind, it also gets worse for the wait staff. Since the venue owner decides he won't pay bussers, bartenders, or dishwashers adequately either, the wait staff then often loses chunks of their tips to pay these guys. Who does this benefit other than the venue owner? Consider the example below --
If everyone is excellent and firing on all cylinders, then I guess the wait staff can't argue with giving up part of their tips to these other staffers, despite that I the customer have no idea of the trickledown system at a given restaurant, nor do I care -- my judgment for tips is usually based on wait staff and kitchen (prep) staff performance alone. (Service at this excellent level by all staffers involved is unlikely enough to be ruled out, in my experience.)
What if the wait staff is excellent, and gets great tips, but the other staffers they fluff up with tips aren't, and the wait staff is covering for them? Doesn't matter, they still take a percentage of the wait staff's tips.
What if the wait staff is poor, and gets poor tips, but the other staffers are excellent? They're supposed to get penalized? How does this motivate them to be excellent in their jobs, when their pay is limited by the wait staff whom they have no control over?
The whole system is broken. Wait staff should be paid an adequate wage, with the opportunity to earn tips as bonuses on top of that for merit, which they keep for themselves. The customer shouldn't be treated in an adversarial manner, in which they are propositioned for a guaranteed level of bonus or else risk tampering with their order by staffers.
Service staff should be paid adequately as well (appropriate for the job they are performing), and shouldn't be siphoning tips off the wait staff.