They tailbite for any number of reasons.. we don't "officially" know, but we can make educated guesses..
Halfmoons and the larger fin types are notorious for biting their fins, as we believe the fins are heavy and cause drag and pretty much just annoy them.
Filters can be too strong, causing drag and the fins to be heavy.. so they will bite off their fins to be able to swim with ease.
A nervous habit- sort of like us with nail biting. They are stressed and upset.. sometimes when being shipped from a breeder, a betta will start fin biting while in the process of being shipped. Same could happen when they are placed and shipped in a small cup to the stores.
From a breeder's jar, to a cup, to a tank- their muscles aren't built up a whole lot, they aren't used to swimming as much.. they will bite off their fins to ease swimming.
Some are overly aggressive and will bite their fins when they get hyped up, such as seeing another betta, or a mirror.. will turn in circles biting their fins.
Some say they get bored- but fish don't get bored in the sense like we do.. they can't feel the same emotions we do. That is just placing our human emotions onto them- but if the tank is very sparse, nothing to swim through or hide in, some will bite.. but I honestly think there is something else causing it then "boredom".
Since you got him that way I would say he was stressed through the shipping process and being in the cup.. or he was used by a breeder for a spawn then sold (which is common as most of the time breeders will sell them after using them for mass production) and a female caused the fin damage.