The helmet was, first and foremost, to protect their fragile heads, but it had an added benefit that the Tamarain feared, which is why they had outlawed them.
The Mens'ave could communicate by thought, yes, but they had rigid limitations to this. Only a few miles, at most, in peak mental conditions, and this was if there was no scrambling that affected the brain waves they communicated through.
But these helmets changed that limit.
With this on, even at a distance of ten miles over Akkes, he could still clearly hear his thoughts, even though the vulture wasn't trying to communicate with him. It was like a window in his mind.