If I've torn something out of the in-flight magazine but don't want to take the whole issue, I'll often rip it in half to make it obvious to the cabin cleaning crew that it should be replaced (or at least discarded). Not into little pieces, though.

Perhaps the people you describe are trying to destroy documents they'd shred at home or at the office.

Or perhaps they're just obsessive-compulsive.

In the first case, they're not likely to stop.

In the second, they might start doing something worse!