While I suspect this is a screwup on the part of Bixi, you're making the mistake of confusing bikeshare with bike rental. Don't worry though, you're in good company, including many companies that operate bikeshare.
Bikeshare is meant to be a transportation service to get people from point A to point B. More often than not tourists don't use it that way and even when they do, they would be better served using bikes that are not part of the bikeshare system so they're able to go slower than a commuter would and enjoy the sights on the way, as well as stop randomly to checkout something that might look interesting.
Tourists are almost always better off renting a bike for a day or even a few hours than using the bikeshare system.
Many bikeshare systems would in fact be better served restricting access only to long term members.