The more different icons a person uses (by which I mean, uses some way on the site that causes the icon to be displayed), the more expensive it is to serve icons.

Many people with a paid account don't upload all of the icons they are alloted, and rarely use most of the ones they have uploaded. (Sometimes this is because their icon needs are fulfilled at 15, so they buy the paid account to get the extra icons, but they don't need all of the icons a paid account provides) and sometimes it's because they're actually interested in some other paid account feature (the last time I renewed on LJ, it was because I couldn't deal with not being able to expand comments or edit my own comments.)

In any case, a person who is paying for just the icons is going to (a) never be the sort of person who bought a paid account for something other than icons and (b) more likely to feel like they need to max out their icon slot, so as to feel they're getting their money's worth, especially if the a la carte icons were priced where they'd have to be realistically, as just about the same $ as a paid account.