I think that is because "police" refers to the organization and has its own French word la police, while policiers refers to the people who work for the organization. On second thoughts, police can refer to the plural police officers. "Police are expected to set a good example in upholding the law" so I think it should be accepted.
I am in North America now, and when people say "the police" they are referring "the police force". You would not say ,"There is "a police" walking to the door" and be considered to be speaking proper English. You would say, "There is a policeman (or policewoman) walking to the door". "Police" is used in America the way "army" is.
The army is all over the city.
An army officer asked me a question.
The police arrived here immediately. Some police were within a stone's throw away, which is why they responded so quickly.
A policeman asked me for my ID.
I live in the US, and it would be very common to say "the police are here" if one of more policemen came to our door, or to say "he got stopped by the police" if a police officer pulled a person over. We often refer to one or more officers as if they are representative of the entire force, at least in the areas I have lived.
Because it does not say "LES policiers arrivent". "Les" implies a definite plural noun, as in not just any policemen but THE policemen Because the indefinite article "des" (not "les") is used, then you cannot use "the" but have to drop it. I think if you wrote "Some policemen are coming", it would be acceptable since "des" does mean "some". Otherwise while you would say in English "a policeman" for one but "policemen" for many when not talking about a definite one, in French you would say "un policier" for one and "des policiers" for many, again if not referring to a specific policeman but just officers in general. Unlike English where the indefinite plural "policemen" does not need an article, in French it does require "des".
There is a difference, however, in the sense of one calling the police. I don't care if it is some specific police officers, whether they are men or women, of it there is one officer or a dozen of them. They are "The Police". So in idiomatic English, we will not want to get too stuck on the "des" vs. "les" issue which is very specific in French. Is it "some police officers"? Yes. But idiomatically, "the police" works in English just fine. There is not any confusion that the entire roster of on-duty officers or just one or two officers may be outside your door.
I do agree, that "police officers' can be referred to as "the police". However, in the case where "des policiers" is being translated to "policemen" or "police officers", I believe using the definite article "THE policemen/police officers" would be an inaccurate translation since it would imply specific cops are being referred to, n'est-ce pas?
The liaison after plural nouns is an optional liaison but one that is commonly made, particularly in more formal speech, according to this reference:
An officer is not necessarily a policeman. A soldier in the army is an officer but not a policeman. A naval officer is an officer but not a policeman. What about a CEO? Chief Executive Officer, also an officer. :c)
You would have to specify "police officers" so we know you mean policiers.
There is nothing improper about "Police arrive". It is a perfectly good present tense sentence describing an occurrence.
Gunshots go off. People run in all directions. No one knows where the shots are coming from. People are screaming. Bloody bodies seem to be everywhere. Some people are on their phones as they run for their lives, probably calling family or the police. Sirens can be heard approaching. Police arrive. They appear to have located the source of the deadly gunfire that started the mayhem and return fire for what seems like forever....