When a login is required to view content, how do Google, Bing, and the others crawl the site?
A more comprehensive method is based on ip ranges, say whitelisting traffic from Google and Bing. This gives you > 95% of search traffic as Google alone has >90%, and many various smaller search engines like Yahoo or DDG are Bing resellers.
On the other hand, a pure ip based check can be circumvented too. Sometimes you can view how search engines see a website through google translate. But places like LinkedIn have countermeasures for all of these circumventions.
As usual it's OK for the big guys to break the rules whereas us little site owners have no choice but to obey them.