It is quite clear that Canon has grudgingly users with APS-C when it finds only one prime lens EF-S. Modern cameras like 70D (and the future 7D Mark II) need high quality lenses, and fast primes are lacking, especially wide angle. Although the new lenses 24, 28, 35mm IS are great, are not as light and cheap as can be EF-S lenses. I must admit that in this aspect, Nikon and Sony treat their customers better by offering several options suitable for small cameras such as SL1, that is orphan of lightweight and fast primes.
This is weird reasoning and it is not clear to me that Canon is some sort of at fault here.
Firstly, primes are not lacking. There is an abundance of them, and they're all versatile enough that you can use them on FF too. Fish goes down to 8mm, and 14mm rectilinear isn't too bad either. Actually, I think Canon is doing crop users a favor. Primes are much cheaper than zooms as it is; if Canon were to double their primes catalogue to accomodate EF-S, it would more likely than not *increase* prices. Regardless, if you're buying primes, you've likely invested more than zoom users. And it's not just Neuro's (very valid) point on that, but it is in your best interest too, if you're climbing the ladder of quality.
Because: what quality advantage would crop primes get you? The low light advantages are there, but the 70D is at 12k8 and following camera's will only fare better. That's pretty extreme already. Sure, wider aperture is better, but it's a relatively small advantage, and if you want better, again there's a slew of FF bodies waiting for you. I think it's very sensible of Canon not to allow you to buy (more than one) EF-S prime.
The most important feature of primes by far, if you ask me, is the DOF they deliver. And if you want maximum DOF, you must go FF. If memory serves, FF has more than a full stop advantage. So, if you're buying to get DOF, you can just see EF lenses as upgrade-proof. Whether that is because Canon wants it or because you want it, is immaterial.
I admit that an approx. 10mm EF-S prime could find a niche, but everything else is covered.
Oh, and other manufacturers are *not* being nicer; Canon makes lenses for smaller camera's where that is prudent (EF-M anyone?). And Nikon only has a lot of crop lenses because they forgot to even *make* FF dslr's for about a decade.