So will AITA if I don’t tip?
Yes
Is it really my personal responsibility to make sure my server is paid enough?
Because of the circumstances, taxes, and customs, yes -- in this case -- the responsibility has been pushed onto you.
You're right that it is ridiculous and unfair, but it is also currently the way it is. By not following the custom, the one being most hurt is the one least able to do anything about it. You also have to walk around afterward thinking that you maybe did not do the best thing.
It seems there’s a lot of discussion about getting rid of tipping, but I don’t know how much has changed in this regard.
Some experiments have been tried, and in some places a 'service charge' appears in lieu of tipping (you need not tip in these places).
Also, recently, counter service has put out tip jars and credit-card screen prompts for before-service tipping. You need not do it at all there. However, if a server has served you beyond the counter, tipping after the service is customary.
To tip 20%, take the subtotal (before the taxes), and move the decimal point one to the left. $28.00 becomes 2.800 (10%) then double that result, $5.60 (20%) is a good full-service tip on a $28.00 bill. More is welcome but never expected or required. Tipping down to 15% is fine, too, don't try to make it an exact science. Tipping outside of 15-20% -- after the service -- is usually done to send a message although studies have not shown that servers really care about those messages too much.
Because of this dastardly system, not tipping is particularly bad because tipping is most of their income. By not tipping, they are working essentially for nearly free in most U.S. states (a very low hourly rate, well below poverty wages).
And finally, if you don't know, honestly and politely ask them or a manager. "I'm not from the USA. Can you tell me how much I am expected to tip in this situation?" Servers may be a little generous with their answer, but most people are honest and happy to know that you will not stiff them for their tip.
Supermarket stockers are not tipped so a higher minimum wage applies, which is $7.25 right now. However, laborers are also in high demand so most places pay more than the minimum.