I'd rather address the stupidity of online installers in general since we're on the subject, actually.
I'd agree that it kind of sucked if it wasn't bog-standard industry practice in the present day. I mean, Flash uses an online installer. Java uses an online installer. Adobe Reader uses an online installer. Hell, Windows itself has used an online installer since, what, the year 2000 or so? Any widely-deployed corporate product is going to use an online installer, because it's infinitely more convenient than having to upload a new version of the client every single time you make a tiny tweak somewhere. Really, anyone complaining about this miniscule amount of bandwidth in the year 2014 is most likely living in a place where gaming effectively is the least of their concerns.
I've already pointed out that I have a crappy home connection but can get easy access to better ones so I pretty much refute your entire argument that you can't game on a poor connection. Yeah, you can't play online games sometimes but I don't particularly care for online play.
As for the prevalence of online installers, I've already made it fairly clear that people should be shot for that one.
I think it's quite ridiculous that in order to install Flash on three different computers I'd have to download it three times or spend ages looking through their page for an offline installer (Hint, I've never found the ****ing thing even when I have looked. They've done quite a good job of burying it). Java may use an online installer but you can find a download link for the offline installer on the same page. That's a great solution for the problem for the customer.
I can understand the logic of wanting to make sure the person using your program has the most recent version, and I can understand why the company might not want to support several different installers, but why not simply have the installer download a file which can then be reused if you have to install on another PC?
Why isn't that a standard option? It's not exactly ****ing hard to do and it makes your customers lives much easier. It also saves you bandwidth and therefore money. Hell, why not allow the customer to use an online installer to download but not install on one PC and then move the files to the PC you do want.
Even if you don't have multiple PCs this approach would still be a good one. How often have you been in the middle of an online install and wanted to switch off, reset or do something else with the PC. If you're just downloading you could just say "**** it" and cancel. Most people are far more reluctant to do that when the program is actually installing something. Doing things this way would completely avoid that issue.
I don't particularly want to single out Steam for this particular piece of idiocy. Like you said, everyone does it. That doesn't make it a smart thing to do though.