On the other hand if tipping is paying for service, why do you 'deserve' a bigger tip just because I ordered the more expensive thing? If the 'fair' price for you talking to me about wine for 2 minutes and then bringing me the the bottle I want is $15 then surly you deserve $15 no matter if I select the $50 or $500 bottle.
Between a $15 bottle and a $30 dollar bottle I wouldn't expect a big difference in service level, so double the tip would not be justified. So I can see what you mean, the connection between price and services rendered is a loose one.
Although if I'm in a restaurant with $500 wines I usually expect the service to include a wine educated sommelier to create a good wine pairing depending on my taste preferences and the food I ordered. That is a lot different from just bringing the bottle.
I expect the restaurant to also be a lot cleaner and more detail-oriented with its service and presentation in that price range.
All in all I think tipping in general is wrong though. The chef making my food has the most influence on my experience and at the same time usually doesn't get a tip (I live in Germany currently, so no tipping out here, but even in the US I think they never get that much). I think waiters are obviously opposed to getting rid of tips. They might complain about the lower minimum wage, but the fact is if they're friendly they get a lot more money than for any unskilled job, especially because tips aren't taxed.
A friend of mine in uni used to waitress at a bar. She said her wage is 4€/h. I asked her why she's doing it. She said she gets between 30€/h in tax free tips. That's roughly the net wage of an engineer in Europe (although you don't get the payments into social security, but most people don't think that far ahead). And this was in Germany, a low tipping country. I imagine in the US if you're friendly and fast at serving you can make much more.
I usually expect the service to include a wine educated sommelier to create a good wine pairing depending on my taste preferences
Sure, but most restaurants that have $500 bottles of wind also have $50 bottles of wine and I'd expect the same level of service even if I end up ordering the $50 bottle of wine. So even there it feels like tips should be 'constant' for a given order.
On the other hand if tipping is paying for service, why do you 'deserve' a bigger tip just because I ordered the more expensive thing? If the 'fair' price for you talking to me about wine for 2 minutes and then bringing me the the bottle I want is $15 then surly you deserve $15 no matter if I select the $50 or $500 bottle.