Helicopters are limited to 200-500 agl.
They're not always limited to any particular altitude.
91.119 has some guidelines for minimum safe altitudes, but they don't apply during takeoff/landing. Helicopters can and do land off-airport. Sometimes they land in unusual spots for emergencies (medevac) sometimes for other reasons.
Even when not taking off or landing, 91.119(c) says that, over open water or a sparsely populated area, there is no minimum altitude required, just 500' from any person, vessel, vehicle, or structure. That rule allows any aircraft, fixed wing or helicopter, to legally fly down in "our" airspace below 400' AGL (but only over open water or sparsely populated areas).
But 91.119(d) allows helicopters, powered parachutes, and weight-shift-control aircraft to fly below the limits otherwise prescribed in 91.119. To take advantage of this paragraph, Helicopters must be operated in compliance with any routes and altitudes prescribed for helicopters by the FAA. But powered parachutes and weight-shift-control aircraft can fly pretty much as low as they want pretty much wherever they want.
Finally, even if a manned aircraft is being operated illegally at a low altitude, that doesn't relieve a drone pilot of the duty to see and avoid it. It's like when we're driving a car and a pedestrian jaywalks in front of us, we'd still better swerve and/or hit the brakes.