My best guess without knowing the intent of the proposer of the rule change or what they have seen at dressage shows might be this part of the rule language (italics mine):
“The use of a weighted shoe, pad, wedge, in conjunction with a hoof band or other device or material (commonly referred to as a performance package) placed on, inserted in, or attached to any limb of a Tennessee Walking Horse, a Racking Horse, or Spotted Saddle Horse(each a breed not recognized by the Federation) or any other horse breed constructed to artificially alter the gait of such a horse, and which are not protective or therapeutic in nature, at a Federation Licensed Competition is prohibited.”
Maybe they feel they have seen some kind of weighted shoe to achieve extravagant gaits?
As to the back numbers on jumpers, it was proposed by the USEF Jumper committee so I suppose there must have been some number of problems that prompted the proposal. I can see the lack of back numbers being a complaint from judges/scorers/ingate personnel, particularly if one rider has multiple horses in a class.