I agree with Mooh too.

But it's because, in a way, the capo is making the writer's intent clear that, while he DOES want an Eb in pitch, the inversion created by the capo is integral to the arrangement--therefore it isn't JUST and Eb--it's a new thing--an Eb that is arrived at by capoing at the first fret and playing a D shape.

If that fingering wasn't integral to the arrangement then simply putting Eb would have sufficed.

JH