Because a Muslim youth entered a police station and began stabbing two police officers. One of them, after having been stabbed multiple times, shot him to death. Rioting has followed. If the police officers had simply consented to being stabbed, there would have been no problem, you see.
"Nine days in Slotervaart: Immigrant youths turn to violence in Amsterdam," by Georg Schreuder Hes for Radio Netherlands:
It has been an unusually violent week for Amsterdam's western Slotervaart district. Cars were torched and youths clashed with police on several consecutive nights after a 22-year-old ethnic Moroccan was shot dead at a police station. He was killed by a policewoman he had just stabbed a number of times. The riots that followed reminded Amsterdam's Chief Commissioner Bernard Welten of a major nightmare for Western European cities: violence on a Parisian scale. Every major town in the Netherlands has its share of so-called problem youths, the type of violent adolescents who gang up to terrorise the neighbourhood. Many of them are the children of migrant workers of Moroccan descent who arrived in the Netherlands in the late 1960s and early 1970s. The Dutch called them guest workers, the operative idea being that they would return to their country of origin when they were no longer needed. So nobody bothered to teach them Dutch, or much of anything else for that matter. The guest workers had their wives come over, but they, just like their husbands were not expected, or encouraged, to integrate into Dutch society....
This article from the Dutch mainstream press never considers the possibility, of course, that the immigrants may not have wanted to assimilate, or that they may have a model of society that they prefer to the Dutch one, and eventually hope to impose in The Netherlands.
The situation was substantially exacerbated by successive governments that chose to ignore what people in the streets knew was an increasingly serious problem. For years, political correctness dictated that problems with migrant children could not be openly discussed. Anybody trying to do so would be accused of blatant racism and Islamophobia. So by the time the social climate began to change, in the late 1990s, the situation had spun well and truly out of control. Violent incidents
Slotervaart, a district in the west of Amsterdam, is not very different from any other poor district in the Netherlands' major cities. What sets it apart from similar districts is a recent series of violent incidents. On 11 October, a 16 year-old Moroccan boy died of a stab wound he sustained in a fight with a 14-year-old classmate after the two got into an argument over a pen. Only three days later, a disturbed Moroccan man walked into the local police station and, without any provocation, stabbed and seriously injured a police woman and one of her colleagues. The man was shot dead by one of the officers.
In the following days, local Moroccan youths torched four cars in the district and smashed the windows of the police station. Since then, police have been intensively patrolling the district, which led to the arrests of eight youths who were detained when officers found jerry cans full of petrol in their vehicle. In the past few days, senior police officers have been making comparisons with the riots that raged in Paris in which more than 9,000 cars were torched two years ago.