Keurig coffee makers have DRM so only approved pods will work. Is it legal to circumvent this DRM and use "unauthorized" pods?
TL;DR: Yes, it's legal.
- There is no patent nor copyright infringement when consumers use third party coffee pods or modify their appliance to accept those pods.
- There may be antitrust infringement when companies take steps to block consumers from using third party consumables like coffee pods.
It's not just legal, Keurig paid $31 million to settle a lawsuit over claims their attempts to block people from using third-party pods violated antitrust law.
The Supreme Court confirmed consumers' right to use a product however they like once it has been purchased in a very related 2016 case about printer cartridges:
The Supreme Court decided 7-1 in favor of Impression on both counts, ruling that once a company has sold a product, it can’t dictate how the product is used—meaning that consumers have free rein to refurbish, repair, or resell items they’ve lawfully bought. “The purchaser and all subsequent owners are free to use or resell the product just like any other item of personal property, without fear of an infringement lawsuit,” Chief Justice John Roberts wrote for the majority opinion.
According to Fortinet.com Cybergolssary:
Digital rights management (DRM) is the use of technology to control and manage access to copyrighted material. Another DRM meaning is taking control of digital content away from the person who possesses it and handing it to a computer program.
As you acknowledge in your first comment, there is nothing copyrightable about a coffee pod. Nor is it digital content, therefore DRM is irrelevant.
Kuerig, as well as other companies, market a product that uses proprietary consumable components of a form that only their product will work with. However, there is nothing illegal about you circumventing their attempt to sell you more of their coffee. It is your coffee maker, do with it as you please...