I have bought a copy of One Piece, but I have to use some apps to read that while I have bought it with a full-priced. For example, I bought a manga online on some random online bookstore. The price for the digital version of that manga is about $7, and for the physical version, it costs $10.
It's understandable/acceptable for the price, with $3 different, because probably it was a cut-off from the printed cost which is come from the printed version, the digital version is print-less...
However, why do I have to read manga on some apps that were used for reading manga online? Why can they just give us, customers, the PDF version of the manga?