This is originally how Memrise worked. Courses were references to a database entry that people could contribute/improve upon but it failed to scale to large amounts of users and especially failed when the language maintainers found themselves too busy to update 'cards'. I used to volunteer my help in maintaining the Japanese/English databases. The largest issue was curation - many duplicate entries would exist due to…
If I understand correctly, the Memrise data is closed source right? So what motivated you to contribute to something you don't control? Or do you keep ownership of your contributions somehow?
A very large portion of the "original" Memrise community were people who had been abandoned by smart.fm, a free-to-use site which later became iknow.jp (subscription model). Our motivation for helping was a mixed bag of mostly the following three reasons, though I don't pretend to have known every original contributor or their motivations.
1) For our own educational use
2) Because we wanted to be able to help others learn our language or a language we had interest in
3) Because we wanted to see the smart.fm community stick around in this new home, Memrise
Personally, I fell mostly under the first reason. I never found Anki good to use and preferred having a website to navigate to and easily sync across devices (even in the early days Memrise had plans for the app) by well... letting someone else handle things. So smart.fm/Memrise were more convenient for me and my learning.
I've played some part in many of the Japanese courses with 50,000+ users. So the work I contributed has helped at least some portion of them begin their studies and I take satisfaction in being able to have done that for them.
E:
I didn't answer all of your questions, so in short. Yes, their data was closed, but they were willing to share large portions of it (except user data, even anonymized user data!). My contributions have my "by {Username}" for my courses - although many of the courses I built/contributed to became standard "Memrise" courses under their name I don't mind getting or not getting the credit - so long as they continue to help people learn!