There is a pretty clear line between fan service and milking a franchise.
Fan service makes money supporting/extending the original experience.
Milking exploits, dilutes, and devalues the original experience.
Rowling has generally taken care to stick to spinoffs that add value; the movies and videogames let fans get a different look at the stories, the candy and the amusement park let them "enter" the fantasy world and live in it.
These are all things that have a justification beyond making money.
Not that there is anything wrong with making money off a quality product.
But some franchises do extensions and spinoffs with an eye to making money more than to enhance/extend the original product. Fan exploitation instead of fan service.
Don't need to go much further than PIRATES OF THE CARIBBEAN...
A clear money-grab from the beginning, the first movie at least had charm and wit and a fun story to go along with the stunts and scenery chewing. The sequels progressively amp'ed the stunts and effects at the expense of character and story until the latest decided to dispense altogether with the central narrative of the first trilogy.
Lots of movie franchises work that way, cranking out episode after episode all built on the premise that if people paid to see it once, they'll pay to see it twelve times with minimal window dressing.
Unfortunately, the premise is true.
