They do it because they like to make you whine and cry. It is a little early for April fools' jokes, I know, but you shouldn't be so naive to see this.
Or maybe these people should just stop playing FotM champions like sheep. Don't get me wrong, there are plenty of people out there who are playing these champions regularly before they became FotM but the vast majority of them are FotM sheep.
Protip: by the time a champion becomes FotM, they're already being looked at by Riot to nerf. Find champions that are strong and out of the spotlight, and you'll be able to avoid nerfs for a long time.
