Yes it seems totally arbitrary. When I first visited the US I paid for our group, and didn't tip the waiter because he got our order wrong, and was met with aghast faces. I didn't realise you're supposed to tip EVEN when the service is bad!
The employer is shifting the responsibility of wages to the customer (you). It is customary in business to pay a wage even if an employee makes a mistake. The tipout structure of most restaurants, where the server tips out the kitchen and support staff, also collects a percentage of all *sales* from the server's tips, so not tipping results in a server paying your tipout from their tips just for the privilege of serving you, hence the agast faces.
Tipping should be illegal to substitute for pay. Majority-tipped restaurants are almost always predatory and take advantage of both customers and employees in order to further enrich the owners.
