They use polypropylene line because it floats, but has very little resistance to UV degradation, even in black. If it's white or yellow it will degrade very quickly. There's no requirement to even have a line attached on a pleasure boat, so you can just cut the line off, or replace with a more durable type like nylon. Just be aware you could foul it in your props. Having the line attached can actually be counter productive, if you can't turn around to pick up your MOB within the line's length, you'll pull it away from them. May as well not even have a life ring if that's going to happen.
On commercial boats more than one ring is usually required, but only one should have a line. You throw the one(s) without (and/or anything else that floats) when the person first falls overboard, then throw the lined one when you're along side the MOB and ready to pick them up.