Well, a couple of points here:
1) Slashdot does not represent software engineers as a whole in the US. Many SW engineers I've met don't seem to be Slashdot users or care about it.
2) Slashdot seems to have an older crowd, since it started back in the 90s. A lot of SW engineers these days are younger, in the Millenial generation. They're not on Slashdot, they're on Reddit or something else I don't know about.
3) Even on Slashdot, there's a bunch of liberals. Remember the Brendan Eich incident? There were people on both sides of that one here. I do think the extreme right + libertarians outnumber them though.
4) Slashdot is not confined to SW engineers; there's a lot of other engineers here too, and in my experience they're frequently even worse than SW engineers.
I do think you're onto something with the libertarian angle; from what I can tell, it seems that the SW engineers tend to be more libertarian and not as religious, whereas the other engineers tend to be more old-school and religious conservative. However even the religious conservatives these days are worshiping the "free market"; the churches here have all bought into that stuff plus Prosperity Doctrine ("God loves rich people more than poor people, and if you're rich, it's because God has blessed you.").