If the bus operator isn’t allowed to enter the parking lot, they’ll know because the parking lot isn’t on the operator’s assigned bus route. Moreover, most of these parking lots are private property. When you’re on private property, you follow the owner’s rules. Drivers of buses, RVs, and other large vehicles are generally presumed to be unwelcome at a parking lot unless invited by the owner.
I don’t know what the norm is globally, but in the U.S., you’ll rarely find a sign anywhere that says “No Buses” even if buses are prohibited. The reason is that the local transportation department can be pretty sure that a bus operator on their payroll isn’t taking a bus on a joy ride. It would cost money to put up regulatory signs for that bizarre scenario. Maybe Los Angeles put some up while Speed was being filmed?
The closest real-world situation I’ve encountered is that San Francisco has installed signs and lane markings exempting “Muni & GGT” from the Do Not Enter restriction on the busway below. Based on a literal reading of this sign, only those two transit agencies’ buses may use the busway, but other agencies that serve the city may not.
In reality, the authorities weren’t concerned about SamTrans or AC Transit bus operators going rogue. Rather, they wanted to make sure that tour buses and tech company shuttle buses wouldn’t enter the busway unauthorized. So this sign actually translates to coach=no tourist_bus=no and whatever the tag would be for the Google/Apple/Meta shuttle buses.
I don’t see how this is relevant. Presumably you’re referring to a path where the cyclist must walk their bike? That does happen along bike routes, typically when a bike path crosses the road at an intersection. However, that would be tagged bicycle=dismount, which (one hopes) has no analogue in public transportation.
We can’t assume that a parking lot has a long marked parking space if and only if it allows long vehicles. For example, Walmart is famous for allowing trucks and RVs to park overnight at many of their parking lots, but they never mark spaces for oversize vehicles. Buses often lay over at the back of a parking lot, splayed out over multiple spaces.
To reiterate, I realize that bus=yes on some of these parking aisles would be pedantic and redundant, but it isn’t misleading. In some cases, as in the “Chestnut Fields” example I gave earlier, bus=yes is a practical tag for indicating that the parking aisle is designed to accommodate buses; alternatives like turning_radius=* are probably unusable anyways.