A Raspberry Pi starter kit could still be put together for around $60 so you are still paying a lot for the kit, sure you don't get the custom OS out of the box or the books however as you stated, you could put it on a RPi yourself. I totally agree that it is nice to have as a complete kit for young kids (especially if they want to learn how to code but don't know any technical people who help them get it set up), but it is not the amazing bargain computer that it was originally made out to be - It has a very niche market.
If kids are learning to code, they are going to be capable of working a regular OS, be it Raspbian, Ubuntu or even Windows on a refurbished PC. Kids nowadays are extremely quick at learning technology and don't really need a super easy to use operating system. A Raspberry Pi would be potentially okay for the most basic of programming but kids would very quickly outgrow it.
I mentor at a group that helps kids learn to code and where they can work on their own projects, a few kids there started off using Raspberry Pis however they quite quickly outgrew them and are now using regular (albeit fairly basic) laptops. Raspberry Pis are still used, but as a platform to build projects on top of such as little robots.etc, not as their primary programming machine.
The way I see it is that for the average kid who wants a computer to learn to program, for $150 you could easily find quite a nice used laptop (At least a Core 2 Duo, possibly even an older i3/i5). This could handle all their programming work (from Scratch all the way up to Java/Python.etc) as well as be able to do web browsing at a decent speed. It also has the advantage of being a single, portable unit whereas the Pi needs to be connected to a monitor, keyboard and mouse.
Don't get me wrong, the Raspberry Pi is a fantastic bit of kit, but $150 seems to be a lot of money for such limited computing power. The kit has a niche market such as in schools and developing countries, but for the average computer user or kid wanting to learn programming, there are better options out there.