Why is it legal?

Junk Faxes were declared illegal in the Telephone Consumer Protection Act (47 USC § 227) and its implementing FCC regulations ( mainly 47 CFR § 64.1200)
It prohibits the transmission of any material advertising the commercial availability or quality of any product, service or property to any person without that person's prior express permission or request to fax machines

Junk Faxes were declared illegal in the Telephone Consumer Protection Act (47 USC § 227) and its implementing FCC regulations ( mainly 47 CFR § 64.1200)