But he didn't go to a maximum security prison at all, infact he was at Västervik a "säkerhetsklass 2" prison. [2]
"Säkerhetsklass 1" would be a maximum security prison, class 3 is the lowest class and those don't even have walls meaning that a class 2 would essentially be a "vanilla" prison.[1]
Furthermore, the prisons in the nordic countries are among the few in the world which actively do practice rehabilitation rather than just "suck", as OP expressed it. I believe that is the reason the vegan diet was brought up as a big deal - he was treated OK in most other regards, especially when considering how prisons work in other parts of the world. Take a look at this documentary:
Yeah I agree, I am however somewhat surprised that they didn't manage to provide him with vegan meals. I mean it shouldn't be that difficult im sure they already offer vegetarian alternatives.
http://arenan.yle.fi/tv/2428667