it's most likely because you didn't have insurance at all. if you had travel insurance you should have forwarded that to them. (but see below about calling an ambulance that is not needed)
if you don't have insurance you have to pay for everything of course. the surprise in your case comes from the unusual situation that the people who called the ambulance didn't know that you had no insurance, or more likely and you weren't even aware of how your situation is going to be handled.
it's unlikely that anything will happen if you return since the ones issuing the bill would not be notified in any way that you entered the country.
it is also possible that you could have disputed the payment since you didn't call the ambulance yourself (and i assume didn't ask anyone to call them). on the other hand if you had insurance you should have gone to the hospital because apparently insurance doesn't pay if an ambulance is called but not used. so actually, you didn't receive a surprise medical bill, but a bill for calling a service that was not needed (and potentially inconveniencing someone else who might have needed the ambulance, but now had to wait).
however, if you didn't ask anyone to call the ambulance then the bill is inappropriate because the law here is that if you call an ambulance but you don't need it, you pay, but if someone else calls the ambulance without you asking them, and it turns out to be unnecessary, then nobody pays.
since i lived in china i also don't have insurance in europe, so when we were visiting and needed treatment for a burn we had to shop around different hospitals to find out which one charged the least. costs for an ER visit ranged from 80€ to 250€ if i remember, and later we found a special hospital that was funded by a charity for the uninsured were we could go for after care for free. that works because the number of people without insurance is extremely small. mostly foreigners who somehow fell through the gap.