Here’s the reponse I got from Hobby Search when I e-mailed them about this.
Dear Customer,
Thank you for your inquiry and we can understand your feeling.
However, to tell you the truth, there are so many requests of introduction of paypay payment system
and therefore we decided to introduce paypal payment system and did our best to solve our technical problems.
In order to introduce paypal payment system, we must accept some restrictions from paypal and be in compliance with their conditions.
We are hoping this sacrifice will be for the greater good, so we hope very much for your understanding.
Regarding your idea [splitting the site into adult and non-adult sections, and only offering paypal as a payment method on the all-ages side, like erogeshop/himeyashop], we would take into account it.
Again, we sincerely apologize for your inconvenience,
and we deeply thank you for using Hobby Search.
Sincerely Yours,
Atsu HAYASHI
On a different topic, Gyut is in the middle of another discount campaign - at the moment you can pick up a number of eroges for 50% off including Katahane and Meguri, Hitohira.