I think it is important NOT to think about entry into the PS in terms of some monolithic and indivisible cluster of motives that apply to all such persons as you describe.
Some folks pursue it because it is the best compensated work for their skillset, or the most dependable employer, in their region. Some pursue it because they need the benefits programs. Some pursue it because there are few other options for the sort of work they are trained to do or want to do (e.g., can you be a park ranger any other way?). Some pursue it because they have visions of "the public good", or have specific agendas (the environment, public health, education, etc.).
There will be people who could work for an insurance company, an accounting firm, a grocery chain, or a federal agency, and would be doing pretty much the same sort of work in any of those environments, albeit for different sorts of clients and employers. There will be people who work much closer to the political machinery and may acquire a bitter taste, but there will be a great many others who work hundreds and thousands of miles away from, and several layers down from that machinery, and feel no relationship, good or bad, to it.
As much as I like, and frequently refer to, the research literature on "public service motivation", one of the bones I have to pick with researchers and thinkers in the area is that there is a big and diverse constellation of reasons why people become public servants, simply because it includes a huge spectrum of job types and related work-motivations, many of them no different at all from the private sector.