Didn't you know, Amazon owns JSON? They acquired it this week, please update all your Content-Type headers within 12 months otherwise you will be in violation of their IP holdings.
If they use a non-standard version of JSON (for example, one supporting comments, or one with rules about duplicate keys, or any other rule that's not part of the underspecified JSON spec) they should use a custom content type. Something can be valid JSON but invalid AmazJSON and this is exactly how you would distinguish between the two.