JSONP does force you to use eval, because JSONP means "instead of returning JSON for me to parse with a JSON library, return executable JavaScript for me to point at with the src attribute of a <script> element". The string consisting of the argument to the callback is de facto evaled by the user agent (when the executable JavaScript is executed) in order for it to become a JavaScript value at all.

The user agent evals the JSONP response before anything else happens: the server might return the string 'yourCallback({"JSON":"rocks!"});', which gets evaled to a function invocation whose argument is whatever the string '{"JSON":"rocks!"}' evals to. If that string doesn't successfully eval (as per the OP), your JSONP is broken.