No, this is not a bug in Errai. You're using a non-annotated parameter which means you want the parameter's value to be part of the request body. GET and HEAD requests are not supposed to carry request bodies that affect the server's response. In fact, XMLHttpRequest will not send the request body for GET and HEAD requests: http://www.w3.org/TR/XMLHttpRequest/#the-send()-method
You either need to annotate the parameter with @PathParam, @QueryParam, etc. (that's what I'd recommend) or use @POST or @PUT instead, if you prefer using the request body to carry that information.