The difference is a home grown transcription vs. copying a licensed version. Hal Leonard has to play a license for all of those songbooks. If one duplicates and distributes it then it is a violation.
I'm pretty sure they got warnings for copyright infringement at one time. People would take bought tabs and put then into GP format. They use to host those.
BTW it is a good notation app for tabs.
Hard to beat the IK version. Honestly a Mellotron is for nostalgia purposes. How kids listening to music on their phones say this mellotron rocks? Basicall it was one of the early samplers.
Maybe it's me but having Arturia, the popular synths that start with S, opening one of the Korg ones that I have just seem uninspiring. Their eco system is worse.