Please note that the sitemap (sitemap.xml) is not blocked. Google and the other search engines can access it just fine. What is blocked is the XSL file used by sitemap.xsl when viewing it through a web browser only for the purpose of converting the XML site map into HTML for display. That is to say, the search engines (for whom the site map is generated) can access it without a problem and take it into account when indexing your site.
As you said, the correct solution is to allow the XSL file (wp-content/plugins/wordpress-seo/css/main-sitemap.xsl) in the .htaccess maker.
New installations of Admin Tools 1.3.1 or later won't have that problem. The file is explicitly allowed and the .xsl and .XSL extensions are allowed by default. These changes will only apply to new installations of Admin Tools. We never change your .htaccess Maker settings on update as we can't possibly be sure which of these are simply an old version's default and which ones are deliberately set to whatever they are now.