it is a fine point of law that has been argued
It is legal to purchase the kits for use on cows, the sale of kits for anyother purpose voids the exemption under which they are sold and makes them subject to the dea scheduling and consequent legal action. Be smart and take advantage of the current laxity in enforcing the requirements but don't think that at some point if johnny law becomes involved you are going to be able to say, "hey officer, it is legal to buy the kits!" Keep a low profile and currently you should be OK.