FCs get credit points from each of their member's kind of activity - doing fates, dungeons etc. It costs nothing for it's members but is a valuable resource for FC - it levelups and FC owners can use these points to pay for company actions. So, FCs are interested even in "dummy" players who does nothing for FC and play as they play. You can use such company to stop invite stream, but there is very few probability to get from it something else.

PS I wrote about possible motivation only, it doesn't mean that all free companies positions most of their members as a source of credit points and that's all.