When you are under power, your outboard is pushing the transom forwards, which in turn puts a strain on the joint between the transom and the tubes — but a strain that the joint is designed to take.
If you attach a tow rope to the transom, the effect is to pull the transom backwards, effectively cancelling out some of the push of the engine, and therefore actually reducing the strain on the joints.
However, the prop is below the transom and therefore the outboard exerts a twisting force on the transom rather than a straight push. The effect is to push the bottom of the transom forwards and pull the top of the transom backwards, very slightly. The joints are designed to take this force.
If you mounted the towing eyes high on the transom, it would increase the twisting force slightly. However, I doubt that it would be significant compared to the twisting force exerted by the motor when accelerating hard on a choppy sea.
I'd be more worried about ensuring that there was no risk of the tow line fouling the propeller.
One thing I wouldn't do is attach the towrope to the tubes or tube fittings (handles etc.) as these are not designed to take forces in that direction.
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