The studios GAVE the cels to workers. the workers 
then OWN them.

which means that if they sell them, they have a 
right to.

its legal

and its legal for you to own them.

Alot of disney cels were not officially released 
either, but it doesnt mean you cant own that 
Ariel cel that someone got from someone who 
worked at the studio.

officially released means they released a scale 
of them to the public, not nessesarily that 
the "unofficially released" are illegal.

at least that's what I came to understand, feel 
free to correct me if I'm wrong!