Study published in Genitourin Med. (currently Sexually Transmitted Infections), 1993 October; 69(5): 400–403. This study shows that using soap to wash under the foreskin contributes to balanitis.

For 72% of the patients with recurrent balinitis, "a diagnosis of irritant dermatitis was made. In comparison with the remaining patients, they had a greater lifetime incidence of atopic illness and more frequent daily genital washing with soap. For 28 (90%) of these patients, use of emollient creams and restriction of soap washing alone controlled symptoms satisfactorily."  

The tissue under the foreskin is mucosal tissue. The glans and inner foreskin are mucous membranes, the same type of tissue as under the eyelid and inside the mouth. Washing the mucous membranes with soap irritates and dries out the tissue. Clean water is all that is normally needed to clean inside the foreskin.