Well if someone were to ask for donations in exchange for features, etc. or said that they were needed to implement certain things then that would be a direct link and so wouldn't really be a donation any more but effectively a commercial payment or close enough to it that it would be against the rules so the moderators could ban the user responsible, or whatever, if it was reported to their attention (and don't forget that enforcement of all these rules is going to be largely down to things being reported or being blatantly obvious anyway).
You don't need to directly demand payment for features, tho. You just cobble together a pretty cool-looking front page, stick in a 'shopped map, and then put 'coming soon' in the download area. Then you stick a donate box at the bottom. More than one mod has successfully used this strategy.
On top of which anyone silly enough to make donations to someone just asking for money in exchange for lots of promises is being pretty damn naive anyway. Something like allowing a link to a tip jar next to the links to download the mod doesn't seem that unreasonable however.
Yup, and you have to be even more naive to believe that the Crown Prince of Nigeria wants to put $50,000,000 into your bank account, yet people fall for phishing emails all the time. Asking for donations in exchange for lots of promises is basically Kickstarter and IndyGoGo's business model, and it seemed to be going quite well (until suddenly we discovered a lot of the kickstarted games turned out to be vapourware, which led to huge outcries of exactly the kind Paradox would probably like to avoid). People do dumb things sometimes, and then get really pissed off when they realise they've been dumb; by banning such things outright, Paradox is attempting to protect forum-goers from that sort of thing, and themselves from angry forumites who've just realised that the site was being used to rip them off.
Besides, if things are going to happen in the wilderness anyway then what's to stop someone creating an external site advertising their mod and with a changelog, list of features, development diary, etc. and sticking a tip jar on there then linking to it in their signature or something? If that kind of thing is going to happen in the wilderness anyway then why not set up some rules (such as tip jars only and only after a working version of the mod has been released or with moderator permission) and allow them in threads for mods here so that community fragmentation is minimised and so that it happens where there's watchful forum members instead of elsewhere?
Because exiling grifters and con artists from the community is a good thing, so who cares about fragmentation when the fragments are bad?
Besides, if something happens in the wilderness, it's not Paradox's problem. No-one will come back howling and screaming and threatening legal action over it. Well, some probably will, actually, but they'll have even less of a legal position if Paradox have nothing to do with the hosting of dodgy sorts. The list is rules for hosting mods on this site; Paradox cannot and will not try and control behaviour in the wild, but this amounts to them taking responsibility for trying to keep their own forum clean of abuse. While they could potentially set up an internal group to evaluate mods and determine whether they're 'good enough' to ask for donations, this amounts to having to a big increase in workload which they don't really need to do if they just say 'no monetization'.
After all, it wouldn't hurt anyone and would certainly be a nice touch until someone finds a better idea for allowing mod creators to get some form of reward for 1000s of hours of work (which would probably only be practical in the form of either building adverts into mods or adopting mods as official DLC which rather defeats the point of them).
It would hurt, tho. It'd hurt anyone who donated to a project which goes down the tubes, or which never really existed. It'd increase the load on Paradox employees when those hurt people appeal to the moderators and community managers over it. Given that maybe 1 in 10 mods is finished, and probably only 1 in 5 actually produces anything close to their mission statement, that'd probably add up to a fairly large amount of extra work - and work that mostly involves telling people 'no, we're not going to do anything about it', leading to angry rants in the forum about how Paradox is a shameful, shabby company for letting this sort of thing happen. I doubt Paradox really want to hire an additional Community Manager on £25,000 a year or whatever so that *I* can make £500 from donations.
While I do hope that Paradox figure out some way to reward modder's efforts, until that happens they're probably right to say 'no monetization'. The first principle of security is to assume people are idiots so you can protect them from both the bad guys, and from themselves. Until you have an idiot-proof, automatic method of letting people show monetary appreciation for a mod, better to keep it away from the official forums.