Could you explain how the term does not apply? Its meaning seems to be: "to impose upon another's generosity or hospitality without sharing in the cost or responsibility involved".
Free software users are voluntarily given use of the software with only specified conditional responsibilities; them using the software and not taking on additional responsibilities is not an imposition of any kind, and not freeloading.
Isn't that the same as when someone offers me a couch to surf and a kitchen to use? Sure, it's unconditional and I'm not violating any agreement, but if I use their stuff and don't give anything back, am I not a freeloader?
But in that case, you are taking up space on their couch and eating their food - their resources are diminished by your use of them. That's not the case here.
I think "free-rider" is probably a better choice of term.