BambooBoy I based my opinion on what I was told by the other contracted authors that I'm talking with on the daily basis, hence my knowledge might be fragmented, not wholesome, but since you want my opinion then let me share.
1st - Why Patreon(Ko-Fi, Paypal and the others possibly) might be censored: This site is providing everyone with free access to hosting your own novels. Additionally, you can fulfil the missions to be awarded free reading opportunities, and if you want to strain yourself a bit and try your best and WPC, you might win not only rewards in form of coins and fast passes but also additional promotion (outside of trending/daily update tabs)
So, to sum things up, you are receiving for free:
- Possible promotion across a site focused on reading novels, but with community different than on the other sites (unlike Watpad, you don't have stories made by teenage girls looking for BL as the sole content -jk)
- Free hosting site for your content
And, there is absolutely no benefit for WN to do it for you.
BUT ADS! - Yeah, let me address this point as well. The great majority of the uncontracted novels are not those whose authors decided to refuse the contract, but those who either didn't fill the minimal requirements (technically speaking here, number of words, collections etc) or were simply... too bad to get one. While I'm not saying it in form of discouragement - my first two novels were trash (not like my next ones are better tbh) but by writing them, I learned how to do some stuff, how an audience reacts, what people want here etc. But that doesn't change the fact, that my first novels were trash.
And that simply means, on average, the uncontracted novel is COSTING WN MORE than it could possibly bring in ads. (considering they won't change the system at all, I'm reading a lot but I have ads turned off so what?). From this perspective, for WN to promote your novels (if you are good enough to get on trending or hardworking enough to get on DU, not to speak about WPC feature) and then just swallow the pill when you either show them the middle finger and just use Patreon and other sites to earn... is kinda wrong, isn't it? Remember, they are not charity!
2nd - Why contracted novels can (most likely) promote their Patreon on their novels?
- Because most of them have other novels that might not be well suited for the WN community. Imagine, creating military spy novel. While it might be insanely well written, WN community doesn't really like it on average.
- Because they can't give any benefits through Patreon - they are still stuck with their CONTRACTED novel fully on WN, but if someone wants to support them additionally through some other means, then why not? I think WN recognises that its easier for some people to just set automatic billing than remember to continuously give gifts.
- If contracted authors were to be deprived of this right, a lot of them (IMHO the part that started with Patreon and only moved on WN and that still have quite a lot of pledges) would protest and force WN to change it back. After all, authors are the content providers for the site, so WN needs to cater to our needs at least a tiny little bit.
I hope that this reply was satisfactory for you, but remember that it's not BY NO MEANS official WN statement, just my intake as to the possible reasons for the situation.