Without everyone, spectators and those involved with the race, attending being tested prior to the event (the WT circus brings lots of people from lots of places with it) it's impossible to know if anyone is carrying the virus.
No, you can't shut down everything but it is, frankly, very easy to shut down bike racing. Yes, it's rubbish for us, towns who rely on the revenue and the teams/riders etc. but in the grand scheme of things it's pretty irrelevant. If they go ahead, there's an outbreak traced to one of these races, it causes the virus to spread and there are deaths it could well be the end of the whole season (at least until a significant decline).