French autorities say that banning away fans for this match had little to do with the safety for Ajax fans but rather the safety for the Marseille fans. "Agressive behavior is something that is seen a lot with Ajax ultras, who mostly operate under the name 'F-Side'. They are known as one of the most dangerous groups of Europe."
In 2009 things already went wrong between the two clubs when Marseille and Ajax faced eachother in the UEFA cup. Ajax fans went with a big group of fans to Marseille. Ajax fans were then attacked by Marseille fans with bricks, sticks and iron bars. Ajax fans then chased Marseille fans until police showed up. In the return in Amsterdam Ajax fans held up a banner saying "We had joy we had fun, we had L'om on the run".
Wouldn't be too concerned about Millwall if they visited Amsterdam. But i don't think Ajax have thr most violent fans in Europe. Our ultras are not angels but compared to a few others in Europe it's not that bad.