If you notice, Garment Printer Manufacturers try to talk you out of buying third party ink for your garment printer. Some will never tell you outright, but let you know how destructive it will be to your printer. Others will flat out tell you that it WILL void your warranty if you use third party ink. Buying third party garment ink is not illegal. In fact, thanks to the Magnuson Moss Warranty Improvement Act, it's actually against the law for printer brands like Brother or Epson to void your printer warranty for buying or using compatible DTG Ink or Garment Printer cartridges.
The Magnuson Moss Warranty Improvement Act is an actual law that protects consumer rights in regards to manufacturer warranties, including desktop inkjet, laserjet, and garment printers. (In case you're wondering, the law's official name is "Magnuson Moss Warranty Improvement Act United States Code Annotated Title 15 Commerce and Trade Chapter 50 Consumer Product Warranties 15 Sections 2302." Here's an excerpt from the law that specifically applies to the use of generic or third party ink and parts in name brand appliances.
"No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer's using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade, or corporate name."
There you have it: using and buying third party garment printer ink is legal and cannot invalidate your printer warranty, no matter what the brand.
Don't let garment printing companies scare and bully you out of saving money and getting better results with your expensive garment printer.
The Magnuson-Moss Warranty Improvement Act of 1975, as well as the Sherman and Clayton Antitrust Acts that precede it, stipulate that a manufacturer cannot void your warranty for using third-party ink or cartridges. It is thus against US law for a printer manufacturer to void your warranty. Warrantors cannot require that only branded parts be used with the product in order to retain the warranty. This is commonly referred to as the "tie-in sales" provisions, and is frequently mentioned in the context of third-party computer parts, such as memory and hard drives.
Federal Trade Commission: http://www.ftc.gov/bcp/edu/pubs/business/adv/bus01.shtm#Magnuson-Moss