Bava Metzia 93a reads: "One who safeguards the red heifer renders his garments impure ... Rabba bar Ulla said: This is a decree, lest he move a limb of the heifer."
But... how does it deter him from moving a limb of the heifer? If the rule was that his clothes would be rendered impure if he touched the heifer, that would be a deterrent. But if they're defiled either way, just as a consequence of him turning up for work, what's the reasoning here?