Cowboy doesn't just refer to a lack of scruples, it also suggests a haphazard, amateur approach.
[eta] I was wondering why we have that usage, so I did a bit of googling. According to wiki:
Quote:
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In the Tombstone area in the 1880s, the term "Cowboy" or "cow-boy" was used pejoratively to describe men who had been implicated in various crimes.[14] One loosely organized band was dubbed "The Cowboys," and profited from smuggling cattle, alcohol, and tobacco across the U.S./Mexico border.[15][16] The San Francisco Examiner wrote in an editorial, "Cowboys [are] the most reckless class of outlaws in that wild country...infinitely worse than the ordinary robber."[14] It became an insult in the area to call someone a "cowboy," as it suggested he was a horse thief, robber, or outlaw. Cattlemen were generally called herders or ranchers.[
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So, I guess we got the usage from there.