Its been said several times already, but I'll add my own two cents.

It's not the same because photoshop doesn't void or nullify any central purpose.

If the central purpose of a sports competition is to select for the most naturally capable human being, then using steroids nullifies or cheats that purpose.

However, there is no central purpose to modelling anywhere which demands that they be natural. We're just looking for aesthetics in models. Not ability, not recognition of natural talent, just aesthetics. Modelling isn't about the model. We don't look at photos of models and go: "Oooh Tyrese Mo' Jackson! She's my favorite model! She's so much better than Tiffany O' Whiteson!" Why? Because, as I said earlier, we only care about the aesthetics. There is no competition going on that the populace as a whole cares about.

If we didn't bother learning athlete names, and we just watched them run / play sports for the aesthetic beauty of the sport, then steroid use wouldn't matter.