Journals that used to sell archive access to individuals for, say, $50 or $100 annually, now won't sell you a membership at all, because to save money on hosting they've moved their subscription infrastructure to JSTOR, and JSTOR refuses to sell individual subscriptions. So you end up "stealing" your access; at various times I've gotten my JSTOR access via ssh -D proxies to a friendly grad student. Now I try to return the favor by providing such access to independent scholars where needed. This sort of gaming shouldn't be necessary with a non-profit organization that is supposed to be working in the public interest, though. Hell, public domain journals from the 18th century that were scanned using public grant money are locked up behind a JSTOR paywall!

For example, the University of Maryland will let you check out books and no more.

edit: although I do agree with your point that in with the current state of things the cost per usage of JSTOR is significantly less than most other electronic journal collections

I work in this field and I can say that almost all databases and journals have IP authentication, generally with the entire campus IP range white listed. If the vendors don't implement that, they generally use password protected accounts. There are some alternatives such as uploading a list of barcodes, Referring URLs (I'm not kidding ProQuest for example allows this), Athens, Shibboleth, and a few others. These, however, are not commonly implemented by libraries because they are not widely adopted by vendors and require additional IT support that libraries simply don't have.

Therefore, unless libraries proxy all their users through their proxy servers (They don't for on-campus users. Usually, all links will go through the proxy, but if on campus the proxy will redirect you directly to the database to avoid the overhead), it would be near impossible to enforce the restriction of guests. I would wager that in many of these scenarios, it's all smoke and mirrors, and that access is really there if you know what you're doing (e.g. just visit the vendor site while on-campus without going through the library links).

And giving it away for free harms the possibilities of being an independent scholar as well. Do you seriously not understand that?