President and chief executive Wayne Pacelle, writing on the society's website, said the Bureau of Land Management was a classic example of how poor implementation could undermine the original intent of legislation.
He argued that the pace of wild horse musters should be curtailed.
"It's never enough to pass a law," Pacelle said. "The agency assigned to enforce the law must work to implement it properly."
Pacelle was referring the the bureau's management of horses under the Wild Free-Roaming Horses and Burros Act, enacted by Congress in 1971.
He said the bureau appeared to be treating it as the Captive Horses and Burros Act.
"The purpose of the Act has always been to protect free-roaming herds of wild horses and burros in the West, for their well-being and as living emblems of the western American experience.
"Yet, despite the intent of Congress, the BLM has shrunk the range of the horses by nearly 20 million acres, rounded up and removed tens of thousands of them, moved them into captive holding facilities, and used its federal appropriations to hold and feed the horses.
"Through the years, it has also stood by as many of the 'adopters' of the horses sent them to slaughter plants for human consumption.
"The Obama Administration has said it will resist any effort to allow slaughter - and that is a welcome development.
"The administration has also pledged to step up the use of contraception - an innovative vaccine and delivery system that the society has long advocated and helped to develop years ago, but that prior Administrations treated as an experimental novelty, not as a practical management tool.
"And it's tried to invigorate the adoption programme, in order to shrink the captive population and the financial burdens that imposes, and talked about setting up horse sanctuaries in the East, with richer grass, that can sustain horses economically and provide a long-term, comfortable and safe setting for them.
"All of that sounds great, and for those elements of the programme, we at The HSUS are on board.
"But the massive roundups and removals of horses must be curtailed. This Administration does not appear prepared to do that - and we are seeing a pattern of behavior similar to prior Administrations.
"The bureau is on track to round up and remove 12,000 horses next year, and that will inflate the captive population to more than 40,000 - well beyond the number of horses on the range.
"And it will add an incredible 25 percent to the number of horses cared for in holding, which the bureau itself has been warning is taking the programme to its breaking point."
Pacelle said he had written to Interior Secretary Ken Salazar arguing that the pace of roundups, including a planned roundup of horses at the Calico Complex in Nevada, cannot continue.
"It is bad for horses and burros, and it is fiscally reckless, with 75 per cent of the entire funding for the programme going to the care of captive horses.
"The upward spiral of spending on captive horse management must be turned around, and the only way to do it is to severely restrict the roundups and removals.
"The original Act never contemplated 40,000 captive wild horses and the use of funds to care for them. If you look at just spending priorities, it's become the Captive Horses and Burros Act, yet the original intent of the Act was to keep wild horses on the range and humanely manage their populations.
"The bureau can do so with the widespread use of contraception, manipulating the sex ratios of herds, and, where possible, expanding the range lands back to their original size.
"While in this crisis, roundups and removals should only occur when the welfare of the horses is in immediate jeopardy or when there is a severe ecological problem.
"And the removals should be carefully monitored to be limited to probable adoption opportunities, at least until the numbers in the long-term holding facilities are significantly reduced.
"If necessary, the BLM, and the taxpayer, would be better served by providing short-term 'sanctuary on the range' programmes that provide supplemental feed and water to herds in the wild, until the contraception programme can stabilise or shrink the population.
"The on-the-range management tools should be used to keep their numbers in check, but most importantly to keep them on the range, and reverse the fiscal and animal care disaster that the current treadmill has created."