Sometimes people just want a really good performing phone that will take a few photos, store some music and make phone calls... Device capacity doesn't matter to everyone, so the iPhone 5c fits that perfectly.
You're talking about 'light' users. I agree that 8 GB might be more than enough for them.
However, for novice users it is difficult to predict whether 8 GB will be enough for them. Furthermore, it is very well possible that using an iPhone makes you want to use it more than you had in mind, thus downloading more games, taking more videos, etc. It is very common for people to say "nah I don't need that, I'll save a few bucks" even though they ended up needing it anyway, because it's hard for them to predict how they will use a device after a weeks or months.
The kicker is, the iPhone is a premium phone. It has polish where you wouldn't need or expect it. An iPhone is supposed to amaze the customer, not frustrate it. 8 GB is certainly going to be frustrating for
a lot of users. Even my mother was complaining about her iPhone 4 having only 8 GB, and she's the prototype low end user who wants an iPhone. Really, she doesn't do a lot of stuff on it, just the basic things, and she has almost zero storage capacity left.
If Apple truly wants to make the best phone possible, they should sell it with
more capacity than people actually need. That way they will be surprised. Even better, they're able to actually use the iPhone the way is was designed, i.e. take HD video, download media from iTunes, download apps, etc. How are users going to engage in the app store if the phone's full all the time? Answer: they won't even bother looking in the app store anymore. They simply stop being part of the ecosystem, leaving little friction for an iOS to Android switch.
Apple's leverage is the ecosystem, users should be encouraged to use that to the max. You simply can't do that with 8 GB.