|It's in the very first sentence.|
Though it is against my nature, I try to be charitable when it comes to assessing the intelligence of others, and this leads me to assume right wingers, Republicans, and libertarians who say the federal government has no business helping people or promoting their well being are wilfully deceitful. Accordingly, people should not trust them. But if they are not liars, then they are woefully uninformed (either because they cannot or will not read the Constitution, however much they might reference it), in which case, people should dismiss their views. What's more, if they haven't read the thing, then they are lying by omission when they act as if they had, which, again, means people shouldn't trust them.
Other possibilities include incompetence (being unable to comprehend the very thing they tout) and cognitive dissonance (in this case, conveniently forgetting something obvious which goes against their overriding views).
In any case, the upshot is, they should not be listened to in these matters.