Yes, you can test negative for a few days after exposure, but fourteen days seems extreme. It's been reported that a majority of people test positive within four days of exposure, though I'm not sure how accurate that information is, since in most cases people don't know exactly when they were exposed.
Italy's announcement caught me by surprise, because Iceland, e.g., allows foreigners to enter if they have a negative test, as an alternative to fourteen days quarantine. South Korea and Taiwan require negative tests before flight to those countries, but even someone testing negative does have to quarantine upon arrival. That seems a little odd to me. If you have to quarantine, anyway, what difference does it make if you have a negative test? Or to put it another way, what do they think a negative test signifies, if they don't trust it?
All the pro sports that are resuming, or making plans to resume, in the U.S. are basically assuming that anyone who is infectious will test positive. I think that's probably true in most cases. To be infectious, you have to have a pretty high concentration of virus in your saliva and other nasopharyngeal fluids, surely easily enough to detect. So while you might test negative following exposure, you probably aren't infectious at that point, either. I guess Italy isn't going to make exceptions for athletes, but this could have been doable if the riders had agreed to be tested every day for a period of time.