Why would it be ridiculous, if a truck has a Bulgarian license plate it's a reasonable expectation that it's based in Bulgaria.
As stated elsewhere: This is politics and does not really change the fundamentals. Change take time. When countries in the EU gets more assimilated, more and more of these restrictions will be lifted.
You ought to see the trajectory and direction more than the absolute position of the legislation.
I've heard that there are long term goals of unifying healthcare, retirement and the way work and local taxes are calculated so that it's more flexible and less based on small local bureaucracy. Similar to what they did with mobile phone subscriptions, where you pay to the country you spend your time in, but can use it anywhere. but that requires a lot more financial systems to interoperate.
As an example, all food in Romania is from the Netherlands or other foreign countries. But you don't see the EU imposing regulations to make foreign food more expensive.
Essentially eastern Europe is an exploitation market: everything is manufactured in the west, and when the east has an advantage it's immediately shut down.
This is also very polluting: forcing eastern European drivers to migrate to their home countries frequently, transporting food from western Europe to eastern Europe.
But this isn't about a Bulgarian person - this is about a truck registered in Bulgaria. That means that the registration fees and taxes, which are a national responsibility and are not at the EU-level, are paid in Bulgaria.
So if you only drive the truck in, say, France, you're essentially evading French taxes for the said truck.
Why?
I don't see any issue with that, it's a win-win.