Because that's not the law. You can't stick a needle into someone without their consent, period. This is not the same as hitting a provisional golf ball after you and your foursome can't agree on the rules. Notwithstanding what the cop erroneously believed, it's established constitutional law. Your solution is not practically different from "let's let the cops come in their house without a warrant and toss it while they're performing a search, but we'll agree to hold the results of that search until later."