They probably just set it to a ridiculous value that also took up the minimum amount of bits and called it good. This would only matter during parsing where you are not taking damage, otherwise you you would be dead quick.
LOL, so while it is not infinite I don't think there are very many builds that can sustain a 20Kps health loss and still do a reasonable amount of damage.
It would probably be fun to have contests to see who can leave it active the longest.