The LP spec doesn't say "the LP request is not a JSON RPC request" explicitly. If it did, then, yes, I could get away with doing GET. Receiving a JSON RPC response is questionable at best in the case that this is true (although normal JSON over HTTP obviously is GETtable).
I don't particularly like the idea of mixing JSON RPC and non JSON RPC in the same protocol. If the LP spec says that you don't need to send a JSON RPC request, then the LP spec is wrong.
I agree. It's not much of a spec. It's more like, 5 half-assed written paragraphs. But that's fine. We do with it what we can.
Perhaps the problem is you shouldn't consider LP as JSON-RPC. Maybe it's not. After all, if it was, it would not specifically mention GET. It's just a hack. I don't see any other way to interpret it. It's a GET request whose response is a JSON-RPC method response packet, completely outside of the normal JSON-RPC flow.
It should also be noted that no matter what you POST to it, it works. You can post completely arbitrary data, and it will still respond with a JSON-RPC response. Or you can post nothing. It is clearly not JSON-RPC.