If you are an ISP, you don't want your clients to spread or become infected with viruses, do you? No. So here's why placing a quota on your clients (though it makes commercial sense) is stupid.
Frequently, Microsoft and Apple release security updates and patches. Sometimes, these large fixes come in packages or service packs that are a good portion of 100 megabytes. I am a conscious consumer, and don't want my computer to be infected or crash. It is only common sense. So I download the updates and use up my internet quota for the month. My ISP charges me for any overages.
When you buy a computer, and consequently an operating system, you agree to a license agreement that says the software is probably not perfect, and that it is not your problem since the manufacturer isn't selling it to you -- they're leasing it. As a result, they feel obliged to fix their software when problems come up (or they want to add features), so they release service packs for "free."
For people with broadband, this isn't a problem. It takes a little while to download the updates, but then you're back to normal. For people who are on dialup (agnony!) or have to pay per megabyte, this is effectively a service charge. You might as well have taken your computer into the shop and said "fix 'er up!"
Cost Analysis. An example ISP here in Oz sells internet access at the rate of $40 for 500MB. That's about 8 cents per megabyte. The last Quicktime update was 70MB. It cost me $5.60 to download the "free" software. I think that the ISP should pay a portion of that to the software manufacturers who release patches. They're drumming up business by automatically insisting users install the updates, and sometimes even automatically delivering them.
If the megabyte-meiser ISPs utilized a caching proxy mechanism where they didn't charge for the updates once they had a cache of them, it would be less filthy -- but the first (probably most conscientious) upgrader gets punished. The result: people realize what's going on and either (1) bite the bullet and pay for the upgrades or (2) punt, and risk infection.
The same goes for Virus Definition files and anti-spyware tools. The act of downloading files that are actually helping the ISP reduce the amount of unwanted traffic (i.e. virus, zombie-creation, spyware, hacker prevention) is being penalized. To the ISPs: eventually some of your clients will become wise and switch to a restriction-free broadband provider; others will bog down your network with virus-laden computers until their systems become useless -- then they will simply discontinue their subscription. Not all 'net traffic is equal; you are not mobile phone carriers; this is not the 1990s. Internet technology is inexpensive -- treat it that way.