A good search engine shouldn't process JS at all, and index only the content that is reachable via links pulled from HTML (which may, of course, be dynamic).

The processing of JS explains a lot of the sheer garbage that is in the Google results.

I think they do it because it ties into their business model. JS is needed to unearth the kind of crap that a certain segment of the user base is looking for. That certain segment is worth catering to because it consists of users who are likely to click on ads.

Otherwise, nobody in their right mind would integrate JS into a crawler. "I'm going to write a program that automatically finds random source code, much of it malicious, and runs it!" Come on; without a business agenda justifying it, it's a complete non-starter.