Uber is not illegal, and the sign is a fake. As 13901 said, UberPool is. UberPool is the equivalent of a regular Uber in the USA: You don't need any qualifications to be a commercial driver, no license, no fees, no car inspection (except by another Uber driver in no official capacity), minimal background check, and lots of fraud. So UberPool was banned, and only Uber Black, the luxury version is still allowed, where drivers must meet the same training, licensing, background checks, car inspection, and other standards as taxi, limo, and private drivers. So most of the Uber drivers in Italy folded up and quit because they didn't want to meet any standards. The Uber drivers that remain I believe are only in Rome and Milan, and because of surge pricing and high demand because Americans still use them they are usually much more expensive than a taxi or a limo.

For example, Uber is legal in Florence, but you won't find any because the drivers all quit when they were forced to meet the same safety standards and regulations as all other drivers. There was only a very brief period of a few weeks in early 2017 when Ubers were illegal in Italy, but that has not been the case since then.

There has never been a time when a tourist would face a 6,000 euro fine for taking an Uber.

You are far more likely to be ripped off by a taxi in the USA than in Italy. It just seems you are getting ripped off because the byzantine one way streets never allow it to be a straight shot to where you are going, so it always feels like you are getting a tour of the city, but rip off are rare compared to the USA. It's not like just driving straight down Fifth Avenue in NYC in a typical Italian city, so it can feel like you are getting a tour.

Sometimes I get in a taxi in Rome and I know the driver is going 180 degrees opposite from the direction of my destination but it is always for legitimate reasons such as traffic, streets that take you even longer because of all of the one-ways, streets that are closed to traffic, etc. You can take Uber, but it is almost alway going to be much more expensive than any other means of travel in Rome.

There is an app called MyTaxi that works about the same way as Uber. It will get in touch with the nearest taxi and you can see the taxi coming on your screen, and follow it on the app to your destination to make some sense of the route.

It is kind of weird to use Uber in Rome when taxis are more economical and less difficult to use.