I've been trying to find out what the lowest data cap on a plan available in the US is purely out of curiosity, but so far I'm failing. I lived so far out in the woods years ago that I had to use satellite myself, and while it was pretty bad, it was nowhere even close to the point of making a 100mb download a problem. Even the HughesNet site's plan with the lowest data cap is enough to download the SR5 pdf over 150 times in one month. I don't want to say that I don't believe what you're saying, but I'm finding it quite difficult to believe that Comcast had you on a plan with a 25mb data cap four years ago without some kind of evidence.
And while I find this stuff kind of interesting in a general sense, I don't believe that the number of people for whom a 100mb download is a barrier to entry is high enough to base decisions off of.
That's because I wasn't on a plan like that. It's one of the many reasons I refuse to ever touch their service again.
As for that 5GB: You're thinking about the data wrong. It's not for just downloads; it's
all data that gets sent to your computer while you browse. That even includes loading this webpage to read this post. Take a look at the lovely background image of these forums, note how big it is, then note it gets sent each time you move to a different page. And keep in mind it applies for every single image, such as when you stop by a site like Drivethrurpg or BattleShop to download a pdf. Notice how many images they have? Ever inspect those to see what the data size is for each one? I checked; the first image that loaded for me is 31KB. If each image is the same, then the combined 49 images for just PDFs alone use a combined total of 1.5MB just to visit the site. Note this is not counting anything else about the site; just the images of the PDFs. If you add in the page itself (21KB), the banners (225KB for the logo, around 58 for the advertisement at the bottom, 185KB for the other Drivethru sections, 26 and 25 for the Wargame and RPGNow banners), an additional 8KP per logo (being really generous here), and you've already used 3.5MB just on the front page. In addition, when you click into the Catalyst section, you're guaranteed to use a minimum of 2.9MB. That's already 6.4MB, and we've not even gotten to the Shadowrun page or the download yet. Let's say you only have to deal with 2MB on the next page (assuming you just go directly into the product itself) and use your figure of 100MB for the download... total MB is 108.4MB just to download, assuming you do nothing else that day; that comes out of a total MB per day of 178.6 (5GB divided by 28 days). That leaves you 70.2MB for the day before you start subtracting from tomorrow's portion.
If you want some fun, assign yourself a monthly limit of 5GB and a daily limit of 178.5MB, then keep track of how much you actually use per day without changing your current surfing habits (remember, you have to keep track of how much data every single webpage you visit uses). Remember that if you go over today, you are subtracting from tomorrow. And, for part of the experiment, let's add to the fun: Every time you go over the monthly limit, you must either slap yourself or stop all internet usage (the slap is to represent the fee for going over the limit). After one day, let me know how far over the limit for the day that you went. Then try to calculate for yourself how many days in the month you have at that rate of usage before you have to choose between a slap or no internet usage. Then keep in mind that a download, such as this, is taking up a rather significant portion of how much time you have (most the limit for a single day) and that you will reach that limit even faster if you download this.
I know you don't feel they're significant enough in number; realistically, they're not. But, at the same time, the issue cited was not to discriminate against those who would rather wait for books, many of which are people for whom a PDF option is not realistic due to data caps. Plus, as Michael pointed out, this also potentially discriminates against stores that actually sell those books. Now, if Bull were to decide it's okay to discriminate against those people, then everything I just said on this topic about data caps becomes moot, since it won't actually matter, what Michael said doesn't matter, and we can move on. All I'm doing is pointing out that, for some people, it really is an issue. Even when they have caps that say that it shouldn't be.