The reason is simple: they'd like to reap all the benefits of a permissive licence (many people and companies won't or can't touch GPL code), without any of the downsides; but these downsides are the very reason behind the rules in more 'restrictive' licenses like the GPL.
This usually doesn't work, and in the end all they can do is complain about behaviours that their license choice explicitly allowed.
This usually doesn't work, and in the end all they can do is complain about behaviours that their license choice explicitly allowed.