Whether a word should or should not be used is complex, and usually a matter of context and fashion. As a society we do sometimes decide (try) to drop the regular use a word entirely due to the sheer weight of the history around its use - the argument being that it becomes impossible to remove the other connotations not matter the context.
So why is one allowed when the other is not when they seem to arise from such similar contexts?
First of all, while one might feel like its use is a 'bigtime no-no', the use of 'wetback' is still very popular in certain segments. So obviously it's only disallowed in certain contexts, e.g. around people who don't hold to that particular kind of racism, in public forums, in professional settings, etc. Among other words it's used every day as a rallying cry of the anti-migrant movement in North America, and as such has a lot of negative power.
'Redneck' on the other hand has been embraced by a 'grassroots' conservative backlash movement, which now has a certain amount of political power. So while originally it marginalized a class of people: the poor rural class that have less access to adequate education or health care, and show many of the same generational problems that visible minorities do; now the term also refers to a whole bunch of definitely not marginalized middle-class Americans. People whose grandparents would never have self-described as 'Redneck'.
These new, political 'rednecks' backlash against anything they consider liberal, including efforts to help or defend the marginalized they purport to represent.
So now in some contexts the use of the term is not only allowed, but encouraged.
But where is that liberal society that's supposed to be defending the marginalized against bad words? Well, the liberals are hypocrites, just like most everyone else in this world. They'll jump on any chance to derogate 'the other' as long as it's fashionable to do so. They certainly won't come to the defense of the 'rednecks' while it's such a convenient rally cry against Trump voters.
So it's us-versus-them politics at play here, and as in any political game the marginalized get trod on by both sides.