This explains a bit better than I can: THE TRAKEHNER BREED AND THE THOROUGHBRED
But, essentially, it is to keep the breed pure, without resorting to other "warmblood" bloodlines. Breeding to a TB or Arab is not considered "outcrossing". The Trakehner gene pool is relatively small, as well, compared to the other warmbloods, because "outside blood" isn't allowed. Anyway...
My mare isn't REGISTERED Hanoverian, but she IS approved for breeding by the AHS. SO, *if* I were to breed her to an approved Hanoverian, that FOAL could be registered Hanoverian. Most European Warmblood "breeds" (registries) allow outside blood, and are looking for a particular TYPE rather than breed lines, per se. I also have a Trakehner mare that is approved Westphalian. I just happen to only breed Trakehners and Anglo-Trakehners, because it is important to me to keep the breed what it was intended to be. I have nothing "against" other warmbloods, but Trakehners are my breed.
As far as crossing a draft horse with a TB and calling it a warmblood, well...there is one born every day who will fall for that. It's akin to someone breeding "labradoodles" or something else and claiming to have a breed, not a mutt. IMO *breeds* are created through generations of successful matching and culling, looking for specific traits. You can't just take "hot" and "cold" and get warm, but every day I see ads for "warmbloods" advertised. It's sort of like the people who will cross ANY black mare, of any breed, with a Friesian. Just not smart, and, frankly, irresponsible.
REAL Warmbloods, such as a Hanoverian, Oldenburg, etc. are more than just a mix of this and that, even if the bloodlines are a "mix" of blood originally from one region and another. There are breed (registry) standards, and the inspection process provides accountability. Horses cannot simply be bred to whatever and produce studbook eligible offspring.