Why does losing a game at 0 LP = demotion?

Got promoted earlier this season, went on a 3 game losing streak (0 LP) no problem. No demotion and won 4th game and gained LP.

Got promoted recently. Won a game, lost a game...then lost ONE GAME at 0 LP. Demoted. Back to 75 LP on tier lower.

I'm not exactly upset, it just makes no sense. You can go on a losing skid if you get promoted, but watch out if you win a game..

The worst part about the demotion is: You're 2 game removed from promos.