It's not about being "illegal". It's about being honest. When someone says they drew it, that involves a LOT more effort than clicking two filters in photoshop for example. He is, in a sense, trying to cheat curators by implying he did a lot more work than he actually did.

If he said, I took this photo, modified it to look like like that digitally, and I present it to you, that'd be far more honest.