I think it all boils down to what you believe to be the fundamental nature of Evil (or, in a game, the fundamental nature of Evil in the setting). If you believe that Evil is a valid natural state, and that sentient beings have the right to choose to be evil, then using a Helm of Opposite Alignment violates that right.
However, if you believe that Good (or at least Neutrality) is the natural state and that Evil is unnatural, then using a Helm of Opposite Alignment is no different from using dispel magic to remove a compulsion, or using greater restoration to cure insanity.
I think that standard D&D takes a dualistic approach that is more in line with the first view (Evil is natural and valid) than the second (Evil is unnatural and should be cured), given the usually equal but opposite forces of Good and Evil, and Law and Chaos, and given that a Helm of Opposite Alignment works to change Good to Evil as well, and not only to cleanse Evil and restore a person to Good.
Hence, in a standard D&D game, the right to choose one's alignment should be considered as fundamental a right as the right to life, or the right to liberty. Evil creatures may callously disregard them, but Good creatures would prefer not to deprive sentient creatures of them, and would only kill, imprison, or change a creature's alignment if it is necessary to prevent them from harming others.
Of course, it is possible to have a setting that takes the second view of Evil, but in such a setting, Helms of Opposite Alignment probably should not exist in their current form. Instead, there would be Cleansing Helms that worked to cleanse Evil from a creature's soul and return it to its natural state and cursed Taint Helms that would twist a creature's soul and make it Evil.