If this company just let you tinker at their offices so you could learn, that seems fine. If you were producing production-quality work for free, well, that's a more complex problem. In that case, you're devaluing the work itself (giving away for free what would normally be sold). And while it's not going to have a huge effect if just you do it, if this scenario was allowed to become the norm it would impact the paychecks of everyone in your industry. Except the owners, who would love the overall cheaper labor.

And, just to say it: It's this sort lack of labor protections that lead to ever-widening income gaps. If companies aren't bound to pay employees fairly, the middle class gets poorer and the wealthy owner class gets wealthier.