@Jeffro - One more:

I think Marks answer is generally the way he and Matt both feel about the guideline with themes and plugins. In this matter, I think you’ll have to realize that this is the way it’s going to be, despite your best arguments against it. While I still don’t like the fact that I can’t control the behaviour of revisions from the back-end of WordPress, I’ve given up my argument for it. Somethings in the world of WordPress are just going to be the way they are. This is one of those issues.

I don't think we should ever give up arguing against an obviously discriminatory practice, just because it's "the way they are" or because it's "the way Mark and Matt both feel".

The argument that plugins are required to make credit links optional and default to "disabled" because a site may display several such plugin credit links, but a theme is allowed to provide such a link with no user option to disable it is acceptable because a site would display only one such link is asinine.

The principle is user choice - not SEO (theme credit links can be SEO-spam, too, after all), not number of links displayed (if all credit links are required to default to disabled, and must be enabled by the user, then all such credit links would have been explicitly enabled by the user).

The guideline for plugins enforces user choice, by requiring plugins to provide a configuration option for the credit link, and requiring that option to default to disabling the credit link. The guideline for themes utterly ignores user choice, by allowing themes not to provide a configuration option for the credit link - much less requiring that the (non-existent) option to default to disabling the credit link.

There is no reason whatsoever to accept this discrimination just because it's "the way it is".

If Mark and Matt believe any of the above arguments for this practice, then, plain and simply, they are wrong.