Yeah, I had the thought that requiring it might be a little too heavy-handed.
Of course, coming up with some kind of extra on-site reward perk as thanks for choosing to use an OS licence that you don't get if you upload as ARR could be a compromise, though I'm not quite sure what such a perk should entail. Maybe some kind of reduced or eliminated site ads just for authors who pledge to upload their addons under OS licences? (Don't know WoWI's operating cost books so I don't know if this is financially viable for the site.)
Seerah, there is the selfishness argument, but on the flip side, the benefit from FOSS vs. ARR arises chiefly at that point down the line when the addon breaks due to an API change and the author isn't updating it anymore; at that point, yeah, it lets others continue to take advantage of your work - but, in fact they can't do this without (sometimes substantial) effort of their own, since they'll have to put in the time it takes to fix the addon so it works with the new version of WoW. What it really does is just spare the community the extra time it takes to do the same thing from scratch.
And yeah, sometimes we do have the opportunity to use something else. X-Perl seems to have some annoying issues with 4.3 (the most obviously visible one being that I could not get Fonter to work with it at all, no matter what I did, all I could ever get on it was the normal Friz Quadrata - and that seems lucky, as the comments on X-Perl in Curse Client suggest that it's likely to slip even further behind on maintenance), Pitbull's configuration options are pitifully few (the configuration interface that was indicated would be introduced after 4.0.3 never made it to release, making it impossible to customise more than a few of the once many options PB had). The community is simply very lucky that Shadowed Unit Frames has been still maintained.
On the other hand, sometimes we don't have that opportunity. No Cataclysm equivalent to Cartographer has surfaced. Neither is there a Cataclysm capable equivalent to Elkano Buff Bars (although Elkano has indicated the desire to revive it for MoP, and I will be glad if that should come to pass, the simple fact is that I haven't been able to create a UI with fully functional buff bars since my Icecrown Citadel raiding days, and as the latest Curse Client description comments on EBB indicate that doing the secure frames version is NOT an easy thing to manage, it's likely beyond my ken at this point given that while I'm certainly no stranger to programming as a whole, I'm a rank beginner when it comes to Lua and addons). The community has not found an addon developer with the time and skill to code equivalents for them during the present expansion, so those addons became "extinct" with Cata, that functionality lost to us for the time being.
In part, I suspect that the reason such an addon developer hasn't been found may lie in ARR: it certainly isn't a pleasant thought for the developer of a potential replacement that they will have to do everything for a fairly complex addon all over again from the beginning, without (just to be safe on a copyright side) even being able to look at how the current addon is coded as a reference. Instead of being able to build on the resources that the community has accumulated over the years, ARR copyright acts as a barrier, forcing the same work to be performed repeatedly in order to bring old tricks into the new world (of Warcraft). This makes logical sense in the commercial software world, where companies are competing for a share of the profit pie and it would hardly be reasonable to expect the first developers to give their competition a free leg up, but this isn't the commercial software world - this is a game community, where ideally, authors shouldn't feel in competition with one another at all, there's no profit pie, and authors do what they do for the community (even if it's just something you coded for yourself and decided was good enough to contribute, the fact remains that you did think of the community because you decided to give it to them
) - but the licensing system can easily get in the way of the community, as noted.
Yeah, I've heard a bit about other addon sites "harvesting" addons and posting them themselves for ad revenue, though I didn't really think all that much about it at the time. And that's definitely a no-no in the case of ARR addons. But why change your open source licence over to ARR in response? When you sign up for an open source licence, you generally do so with the knowledge that anyone can distribute your software, whether you really like them or not. That's one of the building blocks of the FOSS community (and one of the reasons why Linux has a major industry built around it - as it's not Linux that's really being sold and competed over, it's the support deals that various distributors offer if you pay for the distribution rather than simply rolling your own or using a fully-free distro such as Debian).