Successive Detection Logarithmic Amplifier
The text appears to indicate that current output is desired, so that summing is simpler.  I believe based on the text that the diodes are added to indicate a conversion to current output for the summation.  Note that it specifically states that "the detectors should be current outputs (not simple diodes)" indicating a linear voltage-to-current conversion.  If I'm right, I agree that it could have been explained/drawn better.