Petty and entitled customers get to play god with the servers jobs. But worse, they get to do it anonymously. They don't have to face the person or their boss - just click a button and quietly stick the dagger in someone's back. If someone really has a problem, they should have to go to the manager, and not be given this coward's weapon.
You've got a couple of factors coming into play. "Cowards" isn't really relevant, since the restaurant goal almost anywhere good is great customer service, and by making confrontation a prerequisite to feedback you are just blocking negative (and positive) feedback that would let you optimize for great customer service.
Any competent restaurant wants to be providing great service because competent restaurants calculate the lifetime value of their average customer, and it's really high. (Because customers come back to places they like). Couple that with the fact that it's really surprisingly hard to find good employees, and with the very high turnover rate in most of the restaurant industry. (Not all of it--some places have very low turnover and employees who stay with them for decades).
Tablets give the restaurant a way to get more information about the customer experience. Plenty of customers will pay a bill and even the customary 20% tip but not come back if they feel slighted in some way. Maybe neither the restaurant nor the server knows about the problem to begin with. The restaurant loses business, more customers are hurt, the server doesn't improve, the customer loses a potentially good or great restaurant, and it's bad for everyone.
By adding the feedback channel, you have a chance for the restaurant to fix it. Great restaurants will reach out to the customer and offer coupons or refunds or apologies or other solutions the instant they hear there's a problem. Even decent restaurants will at least reach out to the waiter or staff about whatever the problem was (overcooked item X, drink Y ingredient was not in stock, waiter sneered at me when they overheard me mention something political to a friend, etc...)
If done well, that's a good thing, because it makes the service better.