I don't think it was unforced. I think Piastri decided not to fight the corner that hard, let Max have the position. Piastri had far more to lose, worst case scenerio was he got a hit and ended up with no points this weekend. But instead he's still leading Max by 22 points. He'll be thinking he can cover Norris til end the of the season, it's Max he probably more worried about, especially if that RB has closed that gap on say 50% of the remaining tracks.
Yeh Piastri didn't fight the corner that hard, he couldn't because he had taken such a narrow entry into the corner due to holding off the inside from Russell that this left him with no momentum through the corner relative to Verstappen who had lots of space to brake late into and also the superior line in terms of carrying momentum through the corners.
The error was tunnel vision with regard to Russell and seemingly being too passive on the brakes. It's an understandable and small error though, but it was enough to lose him the race. I guess 'unforced error' may not be the correct term as pressure was applied by Russell. Either way it's something that Norris would get criticised heavily for if he had done it.
I don't think it's a big error or a sign that Piastri can't cut it. I just wish people were more consistent with their criticisms of drivers.
He covers Max, Russell takes the inside line and I don't think he thought Max would go for it, although he should've known better. Maybe in hindsight covering Max was the better option but he kept it on track and was on his way to a safe 2nd or maybe some luck with the undercut.
As I've said above, i really don't consider it some terrible error, just as i don't consider anything Lando did in Miami to be a significant error... but if people are going to be very critical of Norris over small things (granted some of it is fair, a chunk isn't though) then same level of criticism needs to be applied to Piastri.
Drives make mistakes, both Lando and Piastri will make more throughout the season, especially at the hands of Max.
Also, Oscar passed Max fine in Miami.
So did Lando, each driver spent the same total number of laps behind Max.
They are both very good drivers and capable of winning titles so long as Verstappen can't battle on a level playing field.