I got stuck in a wall (rare, but it happens), so I tried to change modes to get out. Instead I get a "you cheated" message, lose levels and money and have to wait to die so I can respawn. Sometimes "cheating" is needed. I do understand the want to prevent cheating, but it is sometimes a necessary evil. Someone may want to check out the map in creative like I do after I complete a map (or not), so the instant switch back could kind of suck there too.
Minecraft is a sandbox, let us sandbox man. Besides, if someone did want to genuinely cheat, it wouldn't ruin your fun, just theirs.