Earlier quoted context omitted.
The reasoning goes that EME is a standard that specifically allows for attaching a closed-source black-box to the browser in a standardized way. This code is not auditable by outside sources, and therefore could be a breeding ground for malware and exploits (standard open-source philosophy that "sunlight is the best disinfectant"---many eyes on open source software minimizes the opportunities for exploitable bugs to…
"It's just the "Do you want to allow Flash to play this cool video?" story all over again." Another comparison to existing tech is Javascript. A lot of sites break if you disable Javascript in your browser, but if you enable it you open yourself to Javascript vulnerabilities.
So, it's similar in the sense that enabling JS increases the potential attack surface, just like installing EME/Flash/whatever does. But at least the JS engine can be examined by whoever wants to.