Repeat after me:
A ‘backdoor’ for law enforcement is a deliberately introduced security vulnerability, a form of architected breach.
Or, if you’d like the more sophisticated version
It requires a system to be designed to permit access to a user’s data against the user’s wishes, and such a system is necessarily less secure than one designed without such a feature. As computer scientist Matthew Green explains in a recent Slate column (and, with several eminent colleagues, in a longer 2013 paper) it is damn near impossible to create a security vulnerability that can only be exploited by “the good guys.” Activist Eva Galperin puts the point pithily: “Once you build a back door, you rarely get to decide who walks through it.” Even if your noble intention is only to make criminals more vulnerable to police, the unavoidable cost of doing so in practice is making the overwhelming majority of law-abiding users more vulnerable to criminals.