Well, don't forget that the vast majority of pedigree dogs go to pet homes, not the show ring so cosmetic issues like marking are really meaningless in that context.
Few, if any, from a given litter are potential show quality or worth keeping in a breeding programme for a show breeder. Cosmetic issues really have nothing to do with the quality of a puppy and they are not undesireable in any way at all or considered defective per se -- they simply don't fit show standards and this ONLY matters for a show dog -- so who cares?
Almost none of us who own cavaliers got them to show them. And actually, cosmetic elements like markings -- whether a dog has a narrow blaze for example, or is more heavily black for a tri than broken up with white, are absolutely, the
least important element of whether a dog shows well and there are many dogs that would be considered mismarked that do very well before judges. There are many more important elements that are considered, and markings actually may not matter at all.
There has to be some sort of breed standard though for people to show dogs and for a breed to have a generally consistent look.
I'm really talking above, more about how entire colour combinations for breeds are fixed in the standards when actually there was once more variation. But that fades into little importance (except when unscrupulous breeders try to sell such differences as a costly rarity) compared to breed standards that set or accept physical features that risk, or worse, cause pain and discomfort and a compromised life to the dog. Sadly there are an awful lot of examples of those across different breeds. Some features that possibly (I'd say, probably!) need to go in cavaliers are smaller size dogs, the short nose and huge eyes and perhaps certain skull shapes. Pressure from Pedigree Dogs exposed caused the Kennel Club in the UK to introduce some changes and for vets to approve the dogs before they are given top awards at shows now, for some specified breeds. But cavaliers were not included on the list.
The fact that so many pedigree breeds have health issues which do not show on a basic vet check, as with cavaliers, has many arguing that more needs to be done and that good health test results should be part of the consideration for many breeds, in order for them to win championships.
There is a strong argument in the UK now to create an oversight body for dog breeding for ANYONE that breeds, as the Kennel Club has not done much over the years to change a certain culture and also only could (but doesn't) set enforceable standards for breeders. The KC will say they cannot enforce anything but simply withholding KC registration for puppies would be a pretty strong incentive for many breeders. There has been an investigative body called the Dog Advisory Council (again, a direct result of Pedigree Dogs Exposed) and there is now a push to have that Council become a statutory body to oversee all dog breeding, with legal powers.