Frost walker on mobs

/replaceitem entity @e slot.armor.feet diamond_boots 1 0 {ench:[{id:9,lvl:1}]}
after i use that all mobs not just those that can wear boots had frost walker but the mobs keep falling in.

I expected only those who could wear boots to be affected but i think that its cool that pigs can have frost walker. I expected that when you stand still you don't fall through.

type /replaceitem entity @e slot.armor.feet diamond_boots 1 0 {ench:[{id:9,lvl:1}]}