Huh, this is news to me! Do you have an example site? *make sure you make them nofollow, we don't want to spread any love to those bastards.
but what difference would this 'external' be making. If you say rel="nofollow", it simply means that the spiders do not need to follow the link whether internal or external. As asked by usasportstraining, can you list the name of some of these sites.. would be interested to see it..
The "external nofollow" only applies to outer links, while "nofollow" works on links to both external and internal pages. Some people don't want PR on their privacy or contact us pages, for example. This was discussed in the Comments section of Matt Cutts blog entry, but honestly I haven't seen official definition of the two terms.
Hm, you can put whatever you wish to the rel attribute. Maybe they are using the external word for their own purposes.