Vlad Drumea learns the right way:
It’s free for non-production use
Just like SQL Server Express Edition, Developer Edition is free, but, unlike Express, it’s only free for non-production purposes (developing, testing, demoing, learning).
If you want to learn more about this, Bob Ward has a blog post that covers this in-depth and answers some questions that people tend to have about Developer Edition.
Back in the day, Developer Edition used to cost money—something like $50, so not much, but just enough to make people prefer Express Edition. Those days are long gone, however.