Ok. I looked it up. Here is the language in question:
"(c) Prohibition on conditions for written or implied warranty; waiver by Commission No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer's using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade, or corporate name; except that the prohibition of this subsection may be waived by the Commission if -
(1) the warrantor satisfies the Commission that the warranted product will function properly only if the article or service so identified is used in connection with the warranted product, and (2) the Commission finds that such a waiver is in the public interest. The Commission shall identify in the Federal Register, and permit public comment on, all applications for waiver of the prohibition of this subsection, and shall publish in the Federal Register its disposition of any such application, including the reasons therefor. "
http://autopedia.com/html/HotLinks_LemonMM2.html
Basically, the way I read it is a manufacturer can not specify the use of their product with another product that is specified by brand. The portion about this portion being null seems to apply only if it is indicated in the terms of the warranty. It does allow for such specification if proper operation of the product can only be achieved with the named part or service product.
The latter could be true except that I doubt they would be able to convince the FTC of this in the case of a fluid. They can specify a specific property in the fluid (meets the specifications of their fluid) but the caveat would be that if noone else provides an alternative fluid, you really don't have a choice. But if someone started to manufacturer a fluid that met the specs then I would think VW has no way to deny such a claim. Still, it would be interested in seeing how the VW warranty is actually worded in regards to this.
The quicker explanation is that VW can require a fluid that meets certain specs. But not their brand or a specific brand unless it can be reasonably shown good reason for it.
Bear in mind Nissans had this issue when matic J was first being used. There weren't any alternative fluids (still not completely sure some of the alternatives people recommend actually work) so people were essentially stuck buying Nissan's fluid.