It's illeagal to have Beast Ball Poipole/Naganadel because they are gift Pokemon. When the Pokemon Company revealed Naganadel, it was in a Beast Ball.
I mean... of course they do. Companies and/or people who make trailers and promotional adverts have to change the rules in order to get footage and screenshots.
There is also the possibility that it was originally gonna be distributed by Beast Ball, but they changed it last minute.