If carriers were to set the limits, then you'd have a personalised limit sent to your phone via carrier settings as is the option to use the phone as a personal hotspot or not.
The 50 and then 100MB data cap is probably some leftover that hangs around since the AT&T times when the paranoia that surrounded the release of a smartphone device that had a web browser that didn't limit you to browsing pages utterly crippled by it's formatting and gave you the first mobile device with a centralized application download mechanism - the App Store, iTunes Store (to which the limit pertains) - was strong.
Who the heck cares how someone blows through their data limit? Not even the networks do.
They have an antidote for that - it's called a surcharge for all the data you've unknowingly used up or an unlimited data plan that lets you use as much as you wish without any consequences.
This limit actually cripples the iPhone and doesn't let it be a fully mobile device, that enables you to use the internet connected capabilities as you see fit.
I would also like to have the option to download an update to an app or just an app per se without resorting to WiFi or having to connect my iPhone to the computer all the time and working around the artificial limit on my otherwise unlimited LTE internet.
The OP is absolutely right, even if this forum is only a way to vent and not post a legitimate complaint with Apple, the 100MB OTA limit should be a toggle within the settings with a fair warning saying you might be charged extra for using up surplus data. You should decide whether you want to burn a hole in your wallet or not.
Why isn't this the case with Android phones? Carriers should be in bed with Google as much as Apple and keeping the "precious" data, that they charge a heft sum for anyway, under control.