The reason is the power setting for this speed in the R22 is typically 17-18″ MAP (even lower in R44). This setting is considered low and produces low tail rotor thrust, and reducing roll tendency in the event of low-G situation encountered during turbulence. more ...
Then, why in R22 POH Limitations Section ?
“Adjust forward airspeed to between 60 knots indicated airspeed (KIAS) and 0.7 Vne, but no lower than 57 KIAS, upon inadvertently encountering moderate, severe, or extreme turbulence.”
The airspeed you fly in turbulance is a range. 60 is the low point 71.4 (0.7 of 102) is the high point. If 0.7 of any other Vne (remember it goes down with altitude and tempurature) is less than 60, you must go 60. However, at 10,000 feet and 30 degrees, you cannot go 60, because your Vne is 57! Hence the one exception, “no lower than 57″.
If you fly at 10,000′, Vne is 57 kts (the lowest Vne on the chart) so although we may be flying along at 57 kts, we should not go any faster as we would be exceeding limitations and likely soon experience retreating blade stall. Lets say we flying along at 53 kts (best climb speed), trying to get max performance to be able to stay up at 10,000….Now we hit moderate turbulence again and we do the math – between 60 kts and 40 kts (57 Vne x 0.7). 40 kts is lower than the minimum 57 kts mentioned and we shouldn’t be going 60kts as that is above Vne, so according to the limitations we should speed up to 57 kts.
I think the reason RHC advises not below 57Kts is because 60Kts is the safe speed due to deadman curves, so they wants you to fly closer to 60Kts. But if you are more than 500′ AGL, may be 53 Kts is better, as it produces minimum tail rotor thrust and will be less roll roll tendency in the event of low-G situation encountered during turbulence.