Seems you don't know what a spoiled vote is.

An idiot that doesn't care can check a box and the vote isn't necessarily spoiled. A spoiled vote is when the ballot is invalid. Meaning too many votes cast, an invalid vote, voting improperly (circling a name instead of checking a box).

It's actually less work to walk in and check a single box than either to spoil or to refuse. If I gave 20 of my friends a ballot and said they had to check one box, most would literally check a box. The number that refused or spoiled would be small. Compulsory voting is similar. Except now we need to spend money on enforcing this compulsory vote. I have found one study that says it has no effect on the outcome, so why should we spend money on it?