Legally, it isn’t. The DMCA (and compatible laws in non-US countries, which those countries have to have or they’re not allowed a trade deal, and not having a trade deal with the US is devastating for an economy) doesn’t require copyright holders to do anything to defend their copyright. It does make it illegal to do (nearly) anything with copyrighted media that you don’t have explicit permission to do from the copyright holder (there are some exceptions, but people generally think they go further than they really do). It also makes it illegal to do (nearly) anything to circumvent DRM, even if you have a legal right to use the thing that the DRM is protecting, no matter how crappy the DRM is and how easily it can be bypassed.

You’re allowed to think that the law is stupid (it’s the DMCA - everyone who looks at it and isn’t a multibillion dollar publishing company thinks it’s stupid), but that doesn’t mean that it’s not the law, and for legal terms like piracy, you can’t just substitute your own definition based on what should be legal if it conflicts with the definition that says what really is legal.

The reason why non-crap DRM exists when there’s no legal reason to make it not crap is the same reason why DRM exists at all when there’s no legal reason to have DRM at all when piracy of DRM-free stuff is already a crime. It’s that publishers think that the more of a hassle it is to pirate things, the more likely people are to buy things legally. Technically, a shareholder could sue a company for using crap DRM that failed to protect their IP, but the company has a decent defence by saying that they felt that intrusive DRM would hurt their reputation with legitimate customers, so not using strong DRM is not grounds to say a company’s been negligent and liable for any losses they make due to piracy.