Plastic female RCA connector

I've got a System Audio Subelectro 175 sub. Has line in and out RCA connectors. Line outs look like normal RCAs. Line in however is plastic but still admits a male RCA plug. Why the difference?
What do you mean by plastic? how is it different from "normal"? If it admits male RCA plug then it is normal female RCA socket with a plastic or ceramic isolator between inner an outer metal. There is nothing wrong with plastic - teflon is a plastic.
OK. But I've never seen a set of plastic RCA jacks. They don't hardly grip the male plugs at all. So loose. Also why would they make them different from the other set of RCA females next to them? I emailed to System Audio but have not gotten a response. The pics of the sub on the website clearly show two sets of normal looking RCA female connectors. Anyway I'm running the thing off the speaker terminals they also provide.
Maybe somebody used an interconnect on them that gripped the females' grounding sleeves very tightly, and upon removal this part of the connectors got yanked out? Not a terribly uncommon occurance.