In the Classical style, there are three correct uses of the six-four triad (or sometimes four; see When, if ever, are arpeggiated six-four chords really "real"?). The logic is that the interval of the fourth, historically a dissonance, needs to be correctly handled when it's between the bass and an upper voice.
So why do these same rules not apply to second-inversion seventh chords, which also have a fourth between the bass and an upper voice? The only difference between the two is that the second-inversion seventh chord has a third above the bass in addition to the sixth and fourth.
In other words, a second-inversion triad must be a passing, pedal, or cadential six-four. Why isn't this also true for second-inversion seventh chords?
Is there a historical reason why the triad is treated differently than the seventh chord? Perhaps an explanation from the thoroughbass tradition?
Or perhaps they do follow these same guidelines, but current theory textbooks don't make that clear enough?