In general, how is social welfare possible if there is rampant unemployment? Where would the money come from? If the police lose the support of the community, their usual reaction is to stand down. If the police were to stand down, wouldn't that make the situation even worse?
To answer your question,
@Dotini, I was talking about the rampant unemployment of degraded city boroughs ("ghettos"), which is an exception, not the rule, in large metropolitan areas (i.e. London's current unemployment rate is at 5.8%). Those neighborhoods with cheap housing sandwiched between high-opportunity areas have always existed, albeit with some significant differences, since the modern city was born during the First Industrial Revolution.
The police losing support of the local communities is very problematic - I think we can all agree that our current social equilibrium is based on the state's exercise of the monopoly on the legitimate use of force, and in neighborhoods which are "left to fend for themselves" crime tends to become increasingly more organized and harder to root out (and its rule more pervasive and tyrnannical, which is what I find most troubling). However, most of the time this is a consequence, rather than a cause, of the degradation of those areas.
Low-income areas receive an influx of population from the peripheral areas of the economic system; those low-income areas, however, produce little tax revenue and, often, little political gains and are therefore progressively more isolated (as an example, Clichy-sous-Bois is the only inner suburb of Paris which is not served by any major motorway or railway; IIRC the closest transport hub is the RER station at Le Raincy, some 3 klicks away). People eventually come to resent this abandonment and develop a mistrust in the institutions (which you may find more or less justified) and, arguably, a survivalist sense of morality (I'll let you decide if this is a cause or a consequence of the formation of crime syndicates and diffusion of small acquisitive criminality - but I think you'll agree that poverty is a strong motivator for both phenomenons nonetheless).
At the same time, police forces which are supposed to keep the peace in those areas (which are composed of people, with their prejudices and priorities) are given less and less resources, and at the same time ordered, through law or policy, to enact more repressive policies to limit the degradation of those areas when it is not impossible to hide it from the eye of the public anymore: think of the "stop and frisk" Bratton's so fond of, or the British "sus laws", or the French police racial profiling in Paris suburbs (which is the ultimate cause of the already discussed Clichy-sous-Bois incident).
And when you have a situation of mistrustful citizens and unjust policing, it won't be long before you'll have to deal with an escalation of force which may lead to a stable situation of police deciding it's not worth the risk, or to riots, or to an alternance of both in a vicious cycle in which each time the police has to increase its presence in an area, this will result in riots sooner or later (again, Clichy-sous-bois is probably the perfect example of this tendency).
Of course, one could think of many solution - each with its pros and cons - to the problem. Poverty can be fought in many ways, and so can the poor, if one is not as inclined as I am to equalize as much as possible the opportunities available to all (which is in itself a legitimate position, albeit one that I'd find abhorrent). But the current solution - ignore the problem until it becomes impossible to do so, then apply zero tolerance policies in hope that they will bring the population of those neighborhoods back in line - clearly doesn't work.
Aaaaand this is all a big digression from the original sub-topic, which is "are slums no-go zones because they're populated by immigrants, or because they are populated by poor people, or something else, perhaps a mix of both?".
