The things I could find on justice.gov and other official websites, maybe there's more in the web archive?

- https://www.justice.gov/archives/opa/pr/google-forfeits-500-...

- https://www.congress.gov/110/plaws/publ425/PLAW-110publ425.p...

- https://www.fda.gov/drugs/prescription-drug-advertising/pres...

edit: Oh it was very likely the Federal Food, Drug and Cosmetic Act that was the legal basis for the crackdown. But that's a very old law from the pre-internet age.

- https://en.wikipedia.org/wiki/Federal_Food,_Drug,_and_Cosmet...

edit 2: Might not have been clear for the younger generation, but there was a huge wave of addicted patients that got treated with Oxycodone (or OxyContin) subscriptions at the time.

I think that might have been the actual cause for the crackdown on those online advertisements, but I might be wrong about that.