Why Just Them?
One of the reasons (excuses!) some people use to justify cutting off a horse's tail, is that, they claim it can get hung up in the harness and become a danger. Perhaps one hundred years ago when horses were used for pretty much all farming and hauling, they wore all kinds of harnesses and had tow ropes, extra chains for loggers, all kinds of thing hanging off the back there. The people who handled them usually used just one hand with another long rope so they could have the other hand free to work with whatever machine or wagon the horse was hitched to.
Docking of horses’ tails was originally performed for safety reasons when they were harnessed for activities such as hauling, logging or pulling carriages (Tozzini, 2003). The rationale given for this was to prevent possible difficulties in controlling a horse whose tail had tangled in the harness or reins (Tozzini, 2003; Lefebvre et al, 2007). However, not all draught horses are docked and the inconsistent application of the practice implies that these horses can be managed adequately without the need to remove the tail. Simple and practical alternatives, such as plaiting or bandaging the tail, are available if necessary.
The Animal Welfare Council of Belgium conducted a review and concluded that tail docking was not necessary for draft horses; consequently it supported a national ban. Docking has also been described as cosmetic in the veterinary literature.
https://www.avma.org/KB/Resources/LiteratureReviews/Pages/Horse-Tail-Modifications-Backgrounder.aspx?PF=1
To offer some support to these findings, I looked at several of the breeds including the Clydes, Shires, Belgians and Percherons, who also worked in harness but DO NOT have their tails hacked off: Friesian, Gypsy Vanner (this breed has an enormous tail...not docked!), Fjord, American Shetland, Standardbreds (the most active in harness…nope, not docked!), Missouri Fox Trotting Horse, Canadian Pacer, Trotters, Finn, Cleveland Bay, Gelderland, Holstein, American Saddlebred, Nonius, Orlov Trotter, Fell, Hafflinger and Highland. There are others…however, these horses can be easily researched so you can see they have their full, beautiful tails.
That argument just does not hold water and, as noted above, docking is "described as cosmetic in the veterinary literature". Horses are just made to pay for too many things to appease their owner's follies.
