It is Apple's decision. With a data plan that's 300MB (which exist), downloading 1 app could use 1/6th of your allowance, without you even knowing. If you increase that limit, that problem will become bigger and bigger. Apple is trying to protect you from that happening.
They should at least give you an option of not using wi-fi if you want to.
I have a 10GB/month limit (not a hard limit - the speed is just throttled to 200KB/sec if I download more than that) and couldn't care less if I download something that is 50, 100 or 500MB to my phone using the cellular network.
That happened after the retina iPad release.
The retina devices normally cause the carriers to up the maximum file size for cellular download. When the iPhone 4 came out the limit went from 10mb to 20mb.
I'm sure apple would not want to restrict it but their original and severely limiting deal with AT&T forces them to. And it's not like they (the carriers) will budge. Look at FaceTime over cellular.