This whole deal is just another example of... Real life happens at Disney too. There's no magical dome over the property that keeps all the bad things from happening. Everyone needs to look out for themselves. We had the stroller thing last week and mousekeeping this week.
Disney could very simply have an envelope on the nightstand or something and have it be a dedicated "tip" location. This way it would become a standardized location for mousekeeping to look for it.
Long story short, if you have valuables, make sure they are secured before you leave your room for the day. Simple as that. Common sense really. My how it lacks from people on vacation (and when they aren't as well!).