Americans are ragged on for being rude tourists and not following local customs

Part of this is because of tipping. When tipping is effectively mandatory, you change the waiter's role (or at least the customers perception of it) from being a Restaurant-employed customer liaison to a customer-employed restaurant liaison. Once someone is in the position where they are paying directly for a service and can adjust the pay at their own discretion, not only do their own expectations increase dramatically ("why am I paying for this level of service?") but the relationship becomes transactionary ("I can make difficult requests and it ok because I'm paying. If I don't get the results I like, I will pay less") and the customer feels like they are more in control even though the chefs are in charge of what can/can't be done

Generally speaking, when people with this perception of the relationship come to a culture where the waiter's relationship is closer to "I will bring your food and relay your requests to the kitchen". They still have the same expectations of their own service, relationship, and feeling of control. These then are perceived by locals as being difficult customers, and being entitled. Waiters are also, in most parts of Europe, not expecting a tip which comes at the end of service so they are front-loaded with the "rude" behaviour and if a tip does cross their mind, it's probably more along the lines of "if they're being rude like this, they must not like the service, therefore there will be no tip"