Interesting. I can understand that CiviCRM would see it is an click-through link.
Although external CSS and fonts is very normal for web sites, it is not for emails.
Both Mailchimp and Campaign Monitor advise you to only use inline CSS.
The reason for this is that it is hardly predictable what different mail readers do with the external css. To have a reliable idea how your email will look for your Mailing subscribers it is better to avoid external css.
I would advise you to follow the email rules as shown on the Mailchimp and Campaign Monitor pages for CiviCRM too.
http://templates.mailchimp.com/getting-started/html-email-basics/
https://www.campaignmonitor.com/dev-resources/guides/coding/
Addition:
Campaign Monitor and Mailchimp do a last minute check and make all css'es inline just before sending. They both have published their inline css tool for us to use. The MailChimp version is the best in my experience:
http://templates.mailchimp.com/resources/inline-css