Isn't it supposed to be like that?

This is a deliberate feature. Forward speed has this effect on real helicopters also.

The issue is covered in many tickets about the certain helicopters ingame capable of this.

Above a certain speed the wind is flowing over the aircraft too fast for the tail rotor to make any difference, this is also why certain other flight hazards decrease around this speed. This is authentic.

Now if you are referring to a specific helicopter such as the coaxial mast then you should say so...that said, devs have already stated that the aircraft won't have their unique functions and features...and you may be happy for that because coaxial isn't all its cracked up to be when you can slice your own blades off.

http://www.simhq.com/_air13/air_427a.html

This is by design...

I can tell you that this is not "authentic". I am a real world helicopter pilot with quite a few flight hours, and I can tell you from experience that your tail rotor has authority at all airspeeds. You can damage the helicopter if you apply full pedal above certain a airspeed, but the helicopter will still behave accordingly. Your tail rotor's primary job when you are above ETL is to maintain coordinated flight. With no authority beyond a certain point, it is impossible to complete maneuvers like a pitch back turn, or a RTT, or even a stop turn due to the limitations.

"Above a certain speed the wind is flowing over the aircraft too fast for the tail rotor to make any difference, this is also why certain other flight hazards decrease around this speed."

What does this mean, what other flight hazards are decreased? Why would the wind make the tail rotor stop doing its job, sounds counter productive if you ash me.

You can disagree with me if you like, but it does not change the fact that the tail rotor authority is legitimately unrealistic.

I have played FSX HTR, and from that experience as a reference I will agree that the anti-torque rotor is underpowered in Arma, especially the Mi48, which has a coaxial design yet have the same yaw authority as other aircraft in the game.
However, this is a flight model that is expected to be improved, so there is no real priority to this issue.
The TOH FM is acceptably realistic, and we can look forward to that.

At 60kph translacional flight aerodynamics generated by horizontal and vertical stabilizers superseed the tail rotor controls. Depending of aircraft desing this could be achieved early or later. Some helicopters have devices that limit the amount of left pedal use at high speeds to prevent damage to the aircraft and crew.
I think that this ingame limitation is more or less accurate.
And like crawl55 said you have tail rotor authority in all the flight situations.

The tail rotor should be sufficient to change the flight path to the left and right at any speed. Limiting the stroke of the pedal at high speeds - serves to limit the maximum permissible changes in the angle of the blades at high speeds.
The blades of the rear propeller, first of all, compensate the rotation of the helicopter body from the main screw, its different modes and different permissible speeds.

In T71748#955285, @Peter wrote:

Some helicopters have devices that limit the amount of left pedal use at high speeds to prevent damage to the aircraft and crew.

Consequently, at some speed, where we are now losing the efficient operation of the rear propeller, the rear propeller blades must stop performing effective compensation work effectively. The hull of the helicopter should begin to be brought in the opposite direction from the rotation of the main rotor ? Or when the helicopter is at high speed, it will be impossible to compensate for the unexpected impact of the side wind?
I understand that accidents happen in some situations, but it is no longer from limiting the helicopter to cope with the emerged external conditions. Or the speed of 60-100 km / h for a helicopter on the verge of its capabilities?

Make effective steering work at speeds.