rel=author is part of HTML5, so you don't have to link it to a Google profile. However Google are saying that if you link it to a Google profile then they may display your picture in search results. They are choosing to use their search engine product to promote their profile product.
This post is more reassuring:
The markup uses existing standards such as HTML5 (rel=”author”) and
XFN (rel=”me”) to enable search engines and other web services to
identify works by the same author across the web. If you're already
doing structured data markup using microdata from schema.org, we'll
interpret that authorship information as well.