The reason why the sample app doesn't come with a purchase plugin or an ad plugin is they are useless for a hello world app.
Do you have any idea how difficult setting up purchasing for either IOS or Android is? If you know enough about how to use them, installation of them is simple, if you don;t know how to use them, and downloading the helloworld app indicates you might be a starter, having these additional plugins simply complicates things.
The reason why the original C program was
#include <stdio.h>
main ()
{
printf("Hello World\n");
}
(and thats the original version) was that it proved that your compilation tool chain worked, It provided that the right libraries got linked, it proved that the C preprocessor worked. It gave the user a sample program that worked. It was simple and easy to get going, nothing complicated.
Now if you do not need the Hello World app then you know how to use the plugin system, you also know that there are more than one purchase plugin, that each have their foibles and that each of them are really difficult and you need a good understanding of a whole range of other technologies. Technologies that are not necessary for a simple Hello World.
Hope this helps,
Rob