Slashdot has a large community of posters, but a community does not hold double standards, only individuals may do so. So, Slashdot has a (fairly large) set of community members promoting the GPL and similar licenses, and Slashdot has a set of community members advocating copyright infringement. But since we do not have any information about whether these sets overlap or not, we cannot accuse anyone of holding double standards.
The sets may be completely disjoint (i.e. have no common members), or they may overlap to an unknown extent. But even if they overlap, there is no "average poster" that can be accused of anything, all you have is a set of individuals holding double standards. Only if those sets are almost the same, you can reasonably accuse the "Slashdot GPL advocates" of double standards. But since we have no such information, no conclusions can be drawn, and there is no point in continuing this discussion until someone actually collects and compiles the positions held by a large number of Slashdot posters, so that we have some real data to discuss.