The ssyntax is a red herring here, since the behavior is the same even if you don't use ssyntax like in my previous comment. I'd also bet that it's the same in common lisp. Just a consequence of macros operating in name space rather than value space. Though this is a kinda subtle consequence of that; I had to run that type example I mentioned before to fully grok what was going on.
The reader parses the comma to make (unquote someobj!field) before ssyntax processing even occurs. I just wrote a long comment to describe the whole process, but this is the main point.
