Exactly - it's not Sound Devices, Nagra, Aeta, Zaxcom, etc. who you would expect to have that kind of functionality.
That said, with the amount of things in the 70D that are software controlled, they probably could do assignable channel routing with a firmware update if they really wanted to. It would actually be a really great marketing / sales point for them to be able to say "assign any input to any output" like the "pro" names mentioned above do. They would be the only one in the price class doing that as far as I know.
One reason I could think of them not wanting to do this is that they might think the customer base who would buy the 70D would not understand how to use a routing matrix, whereas someone dropping the cash on a pro-level unit would most likely have experience with that.
Also on this particular unit, it's simpler to have that external line in mini jack labeled "EXT IN 1/2" and then you know that those are the only channels it could possibly be recording to.