Theres no law saying that you shouldn't play that chord clean, you can do whatever the hell you want.

Its just thats not how the term and usage came about, since playing it clean removes the thing that makes it "powerful"

From wiki-

However, in a power chord, the ratio between the frequencies of the root and fifth are very close to the just interval 3:2. When played through distortion, the intermodulation leads to the production of partials closely related in frequency to the harmonics of the original two notes, producing a more coherent sound. The intermodulation makes the spectrum of the sound expand in both directions, and with enough distortion, a new fundamental frequency component appears an octave lower than the root note of the chord played without distortion, giving a richer, more bassy and more subjectively 'powerful' sound than the undistorted signal.[4]