Except there can't be a bubbles for professional cycling.Perhaps luck plays a role but I think more likely you answered your own question at the beginning - "bits and bobs from riders". This wasn't going on at the TdF. The bubble was maintained at the TdF. It is compromised at the Giro. It seems very simple really.
What the NBA did in Orlando is a bubble. What every cycling race organizer is doing is not a bubble. If the TDF bubble was so impenetrable why did some staff members get COVID there?
Look, Italian police is a joke and I have no problem admitting they're very likely responsible for breaches in the protocols. And RCS has a long way to go ensuring riders safety at every level.
But in a non-bubble environment (which all cycling races are) it will always ultimately come down to personal choices and behaviors, not to protocols. It is really hard to oversee everyone who can potentially get in touch with riders and staff members.
Were fans shouting in the riders faces on TDF climbs not compromising the bubble?