As in all things web, this is related to user agents (specifically IE) having a horrendous misunderstanding of what the Accept header is for. If you google "Accept header IE" you'll see a long list of hits about its particular brand of gnarly.

An example IE Accept header might look like such:

  image/jpeg, application/x-ms-application, image/gif,
  application/xaml+xml, image/pjpeg, application/x-ms-xbap,  
  application/msword, application/vnd.ms-excel,  
  application/x-shockwave-flash, */*

Notice the obvious missing entries like text/html, text/plain etc etc. By default, IE is saying "I prefer all these other things to HTML."

The second follow on here is that if a resource provides two content-types for the same resource, and an accept header matches both with equal priority, then the server gets to decide which representation to return. The first example the author gives is technically correct in this regard. His probing shows that Netflix is being a bit naughty, but not without just cause.

Basically, they're making the assumption, "Anyone that asks for JSON really wants it." which does violate the spec, but given the long history of user agents that have historically broken this spec I can only say that its not a question of whether the spec is broken, but how it's broken.

For instance, the spec mentions in passing that given an Accept header of:

  text/html, text/*

That text/html should be preferred to text/plain even though both would end up with a quality of 1.0. Although there's no actual algorithm defined for dealing with this situation. The entire decision tree is left implicit based on the examples provided.

Anyway, getting in a huff that your browser extension breaks a website that tries to cater to developers while providing reasonable content to people that still use IE is a bit awkward. Especially if you've written a book on HTTP and haven't realized that maybe there's a certain amount of spec breakage to deal with weirdo user agents.