Do most restaurants in Sorrento & Positano include the tip in the bill? We found that to be true when we were in Paris a few years ago.
It varies throughout Italy. You need to read each check carefully to see what is included and make certain you were charged for the right meal and items.
As anywhere in Italy, even if service is not included do not over tip! Italian's round up, maybe add a little more for an exceptional experience (which is handed directly to the waiter), but in the more resort areas like the Amalfi Coast, servers have come to expect North Americans to have their usual deep pockets. It is not necessary! You will never see these servers again and they will not think ill of you for not tipping like a North American. Just round up or leave a euro per person if you must.