Earlier quoted context omitted.
That's a good point, but Heroku has a much lower barrier of entry for people who are just starting out. You don't have to worry about configuring and maintaining a Linux box. Some people are willing to pay a premium to avoid those headaches and get back to what they really want to be doing: application development.
$225 premium!? I mean come on. The article focuses a lot on the learning aspect of these exercises, and setting up your own services should really be a part of it. Especially for a one man operation. Typing apt-get install mongo / mysql / rethinkdb after the droplet is really all there is to it.
I've attempted a number of times to bootstrap up my apps on DO, AWS, docker stuff, etc - all in anger. The Heroku tax is less than the time spent configuring, maintaining and monitoring my own servers.
In economic terms, this is called opportunity cost. That's time I could spent building features or playing guitar.