Well the ampersand indicates a separation in field value pairs as part of a query string in the URL's anyway. Using them in any other way would only unearth a whole load of new issues.
In the last few years they added support for more international characters in domain names (accented latin, chinese, arabic, etc.) - But still no support for additional punctuation like &.
The & character is already used in URL query strings to separate name/value pairs e.g. example.com/mypage.htm?name=John&age=30