The thing is that it's stupid to not allow users to join a server that allows to use some cars they own, just because there are some cars avaiable that they don't own. It's really simple. Bad for the player, bad for Kunos. Nobody wins.
On those terms, of course. But there's only one way to let people download content they haven't paid for and not play it - always online, content verification for every session, no unofficial mods (see the mod for AC's F1 cars to let you drive them 'multiclass'... any type of modding that would allow that kind of chicanery would be offlimits). None of which comes for free.
It works for iRacing and I suppose R3E (dunno how active its online is), it wouldn't work for AC without turning it into one of the former.
The best you can realistically hope for with an open platform like AC is "join servers w/ content you don't own, see placeholders, unable to physically interact w/ those players accurately". Even that's a can of worms; if collisions are different in one direction than the other, one group's going to have an advantage. Better off having all players joining the server be on a level playing ground, which means they need to own all the cars that are on the server.
If you just want to compare your times against people with the dlc, if you just want to see races using the DLC but not drive it, well... there's rsr and youtube.
I will confess I don't understand the attitude of "I want to race against it, but I never want to drive it". There are cars in AC that I never want to drive, but... they're pretty much definitionally only competitive with cars that I also don't want to drive. Maybe it's a brand loyalty thing?