Because the Ainol is one of the "high quality" craplets.
The SoC inside is as good as the Exynos 4210 in the SGS2. The features are the bare-minimum I expect. It makes a good companion to tinker about and familiarize yourself with android.

The Elf2 can be had as low as $115, so that's about half the price as the Nex7.
If you were doing risky stuff, you'd be obliged to get the Elf2... and if you stuff up, you can sell the components (make $20 lets say) and you'll also have a new one. With higher prices you're less likely to tinker about.

For instance, the Raspberry Pi costs between $30-40. It's not designed to be disposable, but the use case warrants it so.

And I'd feel better handing a $115 tablet to a child (<12yo) rather than one that costs $249. And last reason is, there are millions/billions of people who cannot really afford to fork out $250... but something $115 may be theesible, especially when considering the gradually increasing quality of these chinese shipments.