Get real.
Dungeon's Legion was the first game for Lunasoft which was digital only, so the circle's practice of a pre-purchase goodie depending on outlet was stopped. Only two places to buy Dungeon's Legion, so little point in setting up pre-order goodies given how easily it is to pirate them anyway. As for its DLC, it was fan demand which brought that into being. It's also completely different from a cosmetic change like this game since the new DLC heroines bring a new game mechanic in addition to their H-scenes (which is content possessing value that can be monetised for consumers).
Every single professional H-game from Japan has pre-order bonuses just like Lunasoft (who are still an amateur outfit despite their production values). This is not "getting away with it". Different pre-order goodies (poster, in-game content, soundtrack etc.) for different stores is an agreement with vendors to distribute the business and mitigate risks of keeping everything in one basket. It is the very rich and fanatic Otaku who goes to every single outlet selling a game for all of the pre-order goodies, most simply pick the bonus they want and stick with it. It doesn't lessen the experience for them since one missing DLC is not going to be a deal-breaker, unlike entitled consumers from other countries imposing their morals and values. Post #69 mentions some companies which do add extra scenes as pre-order bonuses for certain stores, which validates your point but doesn't incite a consumer boycott for "customer inconvenience" and "incomplete gameplay experience". Plus any non-18+ DLC content has a chance of finding its way onto Youtube for non-consumers to see what they missed out.
Does one buy H-games expecting everything bundled in the way Steam and other Western online gaming stores do with their products? In Japan, the answer is no. You buy something because you want the base game, the extras being a nice bonus.
If you feel so cheated at losing out on the bikini armour, look up the pics for this game on the usual online gallery. They've got the whole collection of CGs for all of the costume skins.
Six years is a long time, even for Japanese 18+ games. Lunasoft gave up on physical editions because they weren't economically viable, but up to the latest game the usual practice was the same as other companies in the industry.