I don't think it's about the money. It's probably about being hauled in front of Congress and being threatened with (essentially) nationalization again and again.
Yes. Political spending during major elections makes up a huge part of advertising revenues so financially it makes sense to err on the side of permissive.
Even general government advertising projects are extremely profitable for online advertisers since in almost all cases the only metric tracked is how much was spent by the project/department not how effective it was so you can get away with showing it to lower valued traffic(high bot chance for example) and still make the same amount.