It is because it is a gutless, underpowered heap of junk unless you fly it solo with low fuel. However, lots of people seem to think exceeding the MAP is fine and use the extra power to make up for poor handling skills.
The reason for turning left is that you normally hover into wind and carry out spot turns or lookout turns with the intention of returning to that condition. As you turn past crosswind towards the downwind position, you are likely to see the rate of turn increase due to weathercocking and may apply considerable opposite pedal to arrest this rate of turn. Not a problem if you are turning left since you would use right pedal to stop the turn - but if you are turning right, you will use left (power) pedal to stop the turn and may see a large spike in Torque (MAP) which may exceed the aircraft limitations.
Only Jetrangers seem to have problems with LTE (due to the size of the TR) and I think the rate of turn required to achieve vortex ring conditions on a TR would be far higher than most pilots would ever achieve in a spot turn.