Ask yourself: Is it legal to download One Piece, either Anime(which have DVDs that cost around $15 for 3 episodes I think) or the Manga, which sells in both Shonen Jump and it's own volumes?
It's no more legal than that. Just a little harder to work around.
But it's defiantly worth it. Honestly i it was only about $400 I'd probably buy it.
So in short: Legal? Not exactly. But you download One Piece, or watch/read it in some way that does not give anyone money. You'll find the only people who buy real Photoshop are businesses.
If you think about it, MS Paint is terrible, and has simple features. Worth every penny you payed for it. How much did you pay? none, it comes free with Windows.
You can't even make smooth lines with it. At least if there's a quality about Photoshop you don't like, chances ar ethere's a button somewhere to turn it off. In Paint? you don't have transperancy, layers, or the ability to go back more than 3 steps.
Can you make realistic looking planets in Paint? no. Can you in Photoshop? Yes, easily.
So no, not anymore legal than watching One Piece, or other Anime through subs or scans. So does it really matter?