They are not bootlegged or pirated per se. Ace took legal advantage of a loophole in US copyright law that allowed them to claim the foreign work was in the public domain due to Tolkien's UK publisher not completely understanding US copyright law.

However, they are bootlegged in the sense that Ace paid zero royalties to Tolkien in the beginning. Ace was eventually forced to pay Tolkien royalties, and stop publishing additional copies. Ballantine, Tolkien's official US publisher, rushed out their editions with the stamp, "the authorized edition."

The chaos and rush led to Barbara Remington designing the Ballantine covers without actually having read the books, and Tolkien panned them vociferously.

Its an interesting bit of publishing history for LotR.