WHY WOULD YOU DRINK IT INSTEAD OF SPILLING IT?!

Politics. If he spills it he's at fault for breaking etiquette. There's no proof that it was poisoned, no one trustworthy to test it and not lie about the results. If MC drinks it, now it's murder of a royal, or at least attempted murder. A clear crime. The drink being poisoned is clear for everyone to see. The Mom can still try to claim ignorance that the drink was poisonous, but no one is going to believe her. There's much stronger evidence now.