maybe they didn't actually pee-ed into the drink.
maybe they were just joking to each other and they happened to
think the name 'slurpee' is funny because it has the word pee in
it. and coincidentally, someone buys a slurpee and it triggered the
funny comment they made earlier.
these things happen. but of course, no one would leave it to chance
and still drink it after hearing such things. perhaps, you could
have approached them and clarify what they meant. if not, you could
always lodged a feedback to their manager.