Actually, this is true. F/As are not covered by United should they get injured lifting passengers bags. But there is a tactful way of saying this, and a not so tactful way. I usually just tell the passengers that I am not allowed to lift bags, that United will not cover it if I get injured...but to ask someone sitting nearby, or walking past if they mind helping. Then I tell them that if they have any issues, to come let me know and we will find someone to help them, or put it in the closet if there is room. I use the "check it to your final destination" as a last resort.