Don't think of them as liquid prisons. Think of them as liquid guidance counselors.
Tumblers. The most convenient way to transport liquid from point A to point B, assuming point A is at the end of one's arm and point B is one's mouth. Without tumblers, all of our precious cold liquids would be left to melt into puddles on the floor. When you think of it that way, these tumblers aren't just convenient; they're all that stands between you and fluid anarchy.