I don't think there's any rule against free trials, even if weaker competitors can't afford to match it (at least not in the U.S.)But that was a promotion for just one album that was exclusive for Apple. The no price dumping rule exists to prevent companies from forcing out weaker competitors. Spotify is not going to go out of business if Apple gives away one album for a limited time.
Apple giving away lots of stuff for 3 months that is not free, for Apple, Spotify and everybody else, could be an anti-competitive move that is illegal.
Now, after the outrage, if the FTC went after Apple for doing this, many people would see the FTC as the bad guy.
Maybe.
Also, I thought price dumping was mainly an issue between different countries, not within the U.S. and there it isn't straightforward (whether something is price-dumping or not seems to be heavily influenced by political clout, but the big countries don't like to admit that so complex and poorly explained rulings are always coming out which make it hard to have any idea what is actually allowed or not allowed.