Actually you're wrong about the "make their money" doing doujins because it's actually the opposite. A lot of these artist do it because they love doing what they do, not because they generate money from it. In fact, ater you take in account the registration fees, printing fees, helper fees, transportation and material costs, they end up losing money. And no, they don't "accept that it's going to be put online", they just don't know it's being put up online. And when they do find out, scanlation groups block IP adresses from Japan so that the makerscan't request their work to be taken off the website.
The reason they are allowed to make doujin off official characters/shows is because they don't generate profit from it, they expand the audience of the show, and the most important part is that they don't take away from the main series. When their work gets published online without their permission and the owners of the copyrighted material have a problem with the circulation, it's the creator of the doujin, not the scanlation group, that gets into trouble.
A lot of mangaka and doujinsha have expressed corncerns over what we do with their work. Some don't even sell to foreigners anymore because their work ends up online. And it's not that they have "only for Japanese people" attitude, it's that the work itself is only special when it's concentrated in one area. Like with doujins, the book that you buy is limited in quantities and will never be mass produced, it's a special kind of item. Uploading it online makes it lose the "specialness" and worse yet it's put somewhere beyond the control of the mangaka or doujinsha.
You really should ask for permission before posting any of this up. How would you feel if you made something, ask for it not to be shared, someone shares it without your permission and then you get into trouble for those re-uploads that you have no control over?