How did you wind up at that conclusion? Being forced to join a website for its
downloads doesn't always necessarily mean the owners will earn any cash out of it.
Unless I am missing something here? In that case, someone feel free to fill me in.
It is part of what Clydefrosch said and if you are paying for more than a toy server you likely have bandwidth, be it total transfer or how fat your pipe is (my connection at home clocks some 60 megabits and you get a few of me having some fun...), to watch out for. Get yourself a direct link from slashdot or something and watch it go out of the door with no benefit to your members/community.
Other reasons include trying to get a few people to sign up and contribute -- they say echo chambers are bad but when the echo you hear is just your own voice then you are running a very poor
blog for yourself. I am not sure how well forced sign ups work (every time I am forced to sign up for such a thing I tend to sneak a look at the member database, it is not good if you want to consider more than raw member numbers vs posts) but if you operate on a kind of spammer logic (my Nigerian prince just needs 1 sucker and all that) then maybe you get something from it, especially if you believe the downsides are minor "acceptable" nuisances (see also bank blocking my card and only thinking they are doing it to protect me, or basically everything the TSA does).
Some people might erroneously believe they are restricted to these too, or do it because they always did.
Now the websites that restrict any and all links to signed in users... those are not cool in most instances. Though those I can live with as they are mostly crap or have stuff I can find elsewhere or already know enough to bypass it, it is people saying [facebook link] and facebook then saying
pack it up, pack it in sign up sign in that drives me nuts.