You are misunderstanding. The bikes are not at all unpleasant to ride (although they are quite heavy, they also feel very sturdy and resilient to all road conditions). They would however be very unpleasant to steal and own, because they are too different from normal bikes. It would be a pain in the ass to park them in the city or service them at home and everyone would recognise it as an out of place rental bike.
By the way, stashing bikes behind locked doors is actually what the "OV-fiets" plan in The Netherlands is doing (they are more like lockers, really: http://3.bp.blogspot.com/-PeUfIZbrzY4/Ti8hHZCsLyI/AAAAAAAAAE...), and it works quite well. As a bonus, you can also rent those lockers for your own bike.
By the way, stashing bikes behind locked doors is actually what the "OV-fiets" plan in The Netherlands is doing (they are more like lockers, really: http://3.bp.blogspot.com/-PeUfIZbrzY4/Ti8hHZCsLyI/AAAAAAAAAE...), and it works quite well. As a bonus, you can also rent those lockers for your own bike.