For historical reasons (not wanting to pay for non white labour) in the US, it is customary by businesses to pay service personal minuscule wages, and shift the responsibility for their wages into the customer.
As such, not tipping in the US is utilizing uncompensated labor, which is highly immoral.
Until the current system is dismantled, you can either tip, or not participate in it. But not tipping and enjoying free labor is abhorrent.
You are not allowed to tip as much as you want in all scenarios. GrubHub currently disallows tipping >350% of the order, but, if memory serves, it used to be lower.
Not saying it's wrong - it probably prevents fraud. But still :)
This isn't quite true, unless you bring various bill sizes to go along with a credit/debit transaction. And even then, it can be jarring for some of the situations mentioned in a negative context here (versus baked into the payment steps).
You know that you can tip as much as you want right? That's why it's called a tip. It's not billing.