Work, as in effort, tasks, projects is opposed to leisure, as in peace & quiet, recreation and relaxation.
Work doesn't just refer to performing a job in exchange for money.
Even the term side-project contradicts the idea of leisure. It implies goals, a plan of action and tasks that have to be done.
No, work is work, side-project is a hobby,it's done for recreation, for example to do some stuff you can't find a reasonable excuse to do at work. To explore and enjoy yourself doing what you love. That's also the only reason to have one..
Doing _ANYTHING_ is work if you look at it wrong enough.