I don't believe other manufacturers have the right to enter this category. Both Ford and Holden spent truckloads of money establishing the category, and they deserve to reap any benefit from the category.
People may complain that there are only two manufacturers in the Championship. That may be the case, but the category has evolved (more though than some people) into a driver and team category, not just a manufacturers category.
There are plenty of people who follow various teams for what ever reason, and the drivers don't necessarily drive the same brand. This is similar to Nascar in the States. Before anyone whinges about Nascar, sit and watch a race and be amazed at the level of skill of the pit crews and the drivers. It may look just like an oval but at speed they are 90 degree corners.
If another manufacturer was to enter, what would prevent them from spending all their resources into one super team. As an example, one Mitsubishi in the field against 25 Holdens and Fords. HRT may be Holden's super team (through TWRA) but if you look at every Holden in the field, they all have Holden sponsorship one way or the other (signified by the windscreen banner).
The category is Ford and Holden. It is designed primarily as an entertainment that is competing with numerous other types of sport such as Union, League, Tennis etc. All of these sport exist at a professional level because of sponsorship.
If you want a true test of driver then it has to be single make, with all the cars prepared identically by the one car builder. Formula Ford and Formula Holden are close to this and provide incredibly good racing - but not big crowds.
In sedan racing, the argument of GTP style racing is better is an argument that is utopian. There can never be true parity in cars that are fundamentally different.
V8 Supercars is probably the best attempt at trying to provide an equal platform for unequal driving ability. It works (even though I cannot stand the fact that Ford haven't won a round this year)