Another few very practical reasons: range time is expensive, and magazine loading wastes range time; magazines themselves are expensive and take up space; and some folks have to travel a long way and carefully plan in order to use a range.
I load my semi-auto mags at home before I go, saving range time for actual shooting. I don't pay by the hour, but many folks do, and time spent on the range shoving rounds into castrated magazines is time and money wasted. It also means somebody is waiting for lane time while someone else is on the lane just reloading mags.
A person can only carry so much gear in a reasonably-sized range bag. If I have 15-rounds mags and I need to shoot 150 rounds for practice, I only have to carry ten mags (given that I won't be reloading them while I'm on the range, as described above). If my mags are limited to ten rounds, I need 15 mags, and they take up 50% more room in my bag. If I'm limited to seven-rounders? I'd need 22 magazines! And what is the cost of 22 seven-round mags compared to 10 normal capacity mags?
My drive to ranges where I can shoot anything I own short of belted magnums is under 30 minutes, but many people have much further to go and much less free time. For them, range time has to be carefully planned, and they cannot afford to spend time loading mags while that clock is ticking. Such a person may not be able to practice sufficiently if all her mags must be of limited capacity--she doesn't have the time to load magazines while on the lane.
<True, some ranges will not allow entry onto the firing line with loaded mags. If that's the case where you shoot, have you ever added up how much range time you pay for while all you're doing is loading mags?>
Magazines don't cost necessarily less because they hold fewer rounds. Limited capacity magazines make range time cost more in time and money, plain and simple. Magazine capacity restrictions are therefore prejudicial against the less financially well-off and anyone who works for a living and/or has an active family.