I took this from your link and believe that it answers your questions as to why these tartans can be called "MacGregor Tartans."

MacGregor Tartan

Many will be aware of an organization called the Scottish Tartans Society based in Edinburgh. On looking at the website one will find no less than 10 tartans attributed to Clan Gregor. Some are described inaccurately and some are attributed twice to a specific area such as Glenstrae. Most of them have never been sanctioned by the Chief. Additionally, the tartans section of www.clangregor.com completely inaccurate and should not be regarded as an authority on MacGregor tartan whatsoever. Why is this? And how does this happen?

It happens because those who can speak with a fair degree of authority on the matter such as clan chiefs and their representatives are not consulted and because of the indiscriminate commercialization of tartan. The more tartans there are, the more that are likely to be sold. A tongue in check extension of this is that a MacGregor could wear a different tartan for each day of the week and have three in reserve for special occasions. Every eventuality is covered! This approach is disingenuous and leads to confusion. It is worth bearing in mind that authority for clan tartans is vested in the Chief by Lord Lyon.

It should be said at the outset that for good historical reasons, which will be explained, only the following MacGregor tartans should be recognized as such:

* The Red and Black MacGregor Tartan
* The Red and Green MacGregor Tartan
* MacGregor of Glengyle or Deeside
* MacGregor of Cardney