I am Indian myself and let me tell you first off that I never call a waiter or any other service-provider anything because I wouldn't know which form of address is acceptable to that person. We can be polite without calling them anything, though you may not have expected that hostile response. We don't really have the right to call an unknown person uncle although it is usually tolerated in India for cultural reasons.
In addition to all the other good answers already written, it is worth considering that the waiter might have interpreted South Asians calling a fellow-South-Asian 'uncle' in a foreign country as unacceptable regional overfamiliarity which might possibly not have been overfamiliar in South Asia. As in (interior monologue)
I am not living and working so hard away from home in this far-off European country to be addressed overfamiliarly by these fellow-South-Asians as 'Uncle'! It is a nonsense... Why, I should get the respect I atleast deserves in this country. No I won't tolerate it! (Typical syntax in South Asian English, please don't 'edit to improve.')
So he said cuttingly:
I am not your uncle.
A second possibility which is a global reality is that people who enjoyed high social status and possibly held positions of bureaucratic or traditional authority in their home country may be 'forced by circumstances' to be service providers like waiters or taxi drivers when they emigrate to another nation. Example: I recently read of a nuclear physicist from USSR driving a taxi-cab in New York. Such a person may already resent their self-perceived change in status and your overfamiliarity exacerbates their feelings.
Also remember that extreme politeness is part of the 'cultured public discourse' of many countries and most of those forms of address in Western Europe don't include 'uncle' -- the waiter may feel you have no right to address him any less politely just because you are from the same region of origin -- This is exactly why we must beware of approaching people from our own region very familiarly while in foreign countries: they may not appreciate it, for their own reasons! Better always to say 'excuse me,' methinks.