Killing mines with your courier should be bannable, but apparently it is not currently. If you're not aware, techies has Land Mines which deal damage when triggered. By using a courier, they can trigger the mines but not take damage from them. This is a glitch in the current hosted version of DotA - all other versions had the courier die when they hit mines. In DotA 2 also, couriers are killed by the mines.
The reason is that it is quite obviously a glitch, so falls under this rule:
Rules wrote:6. Do not purposely glitch or exploit the game.
And it's quite strange that this glitch abuse was allowed to begin with. Just because people do it frequently doesn't mean it's OK - I'm sure people would abuse all glitched as much as possible if they were not bannable.