That's not actually true. If you've ever worked in public services or a department of government, you'll know how many of these are highly restricted by law in what they are allowed to do with their budget, to the point where a significant amount of inefficiency in these bureaucracies comes from inflexible budget restrictions. The problem with government spending as I see it is not that they can do whatever they want with money - they cannot - but that almost all of that money comes from the working class but to a large degree benefits the wealthy and powerful.
If Calif we had such a law that would guarantee the money would only go to the school system. Ok, fine. What they did was reduce the money that would have normally gone to the school system from other source by the amount that this new tax revenue would provide. So there was no actual additional revenue to the schools. Another thing they like to do is expand the definitions of the programs the money is earmarked for so they can redirect the money somewhere it was not intended by the original proposal.
So yes, sometimes takes a little creativity but it doesn't stop them much.