I imagine they do this many times just to fill in the gaps where the original jokes in Japanese just flat-out wouldn't make sense. Because of the differences in languages, puns are impossible to retain intact. Other times the original jokes involve cultural references that Americans wouldn't catch, or jokes that, due to cultural differences, wouldn't be suitable for kids here (like the Sailor Moon squad refering to the size of their breasts). So I imagine that many times, they have a decent enough reason.
Not that it makes the new jokes any less lame, mind you...
You read it... you can't un-read it!