I counted them together because they lived / live in the same region and are both non-Germans.
The Frisians are most definitely Germans AND Germanic, though of a distinct group. Due to their habitat they just kept their cultural identity better then most (if not all) other groups, like the Saxons, Franks, Bavarians, etc. When Tacitus described the Germans, he stated that the Frisians settle on the coast, and nowadays - 2000 years later - they are still here, with their own distinct language and cultural quirks. I currently live in Norden, as Frisian as cities come, though I am not a Frisian myself.
The Northern Frisians settled their areas a good deal after Tacitus, an area that became known as Uthlande (outland). The Danish king tried to collect its toll there but was repulsed (not unlike Feudal lords in other Frisian areas), and the area made independent treaties with other areas or trade leagues for some time.
Here is a map of their areas of settlement around 1250 and 1650 - note that the main difference is the coastline:
http://upload.wikimedia.org/wikipedia/commons/a/a4/JBAM_068.JPG
The North Frisians are still a distinct group in Germany, though you will find that they are not so in Denmark, ie they were either assimilated or moved south of the border after WW1 (though there is sitll a markable minority of "Germans" left in Denmark, as there are Danish in Germany.
Anyway, the main point is that it certainly is wrong to lump Frisians and Danish together in a map of languages or population groups. The political situation in Schleswig and Holstein was, well, complex. The area was never in the focus of the Empire, so the Danish king and local Dukes wrangeled it until it became a pretty complex mess of obligations and loyalities. Before 1864 first the Danish and then the German league oversimplified the issue to their own advantage (guess who won), resulting in the border a bit too pro-German that was then shifted perhaps a tiny bit too pro-Danish after WW1, but overall it never was a serious issue again - not even after WW2 were any amendments made.