There is actually good logic behind giving Teemo a low base movement speed. If it were higher, the movement speed buff granted by Move Swift (Teemo's W) might allow a Teemo to forego boots entirely and use the extra gold to buy enough power to stomp any lane competition.
So why not just lessen the effect of his W and give him more base move speed? Well, the current arrangement makes boots a dramatic purchase for Teemo. Move Swift is percentage-based, while boots grant flat movement speed.