This is a non-issue
In many jobs where the staff have to interface directly with the public, it - they, the public that is - can be very trying sometimes. There are some areas of work where maintaing a sense of humour and a sense of detachment are perhaps the only way of maintaining your sanity.
Hence the medical profession has a particularly grim form of gallows humour (or so I'm told, since I'm not in that profession myself); in my area of work we sometimes call the end users of the IT systems which we design and maintain "lusers." You get the idea. It's more of a coping mechanism, and is very, very rarely meant maliciously. Indeed if malice is intended, then I could certainly come up with something a bit richer than "luser."
So if those who help to keep us firstly safe, and secondly fed and watered while we're on board, want to call us SLF when we're out of earshot, then that's fine with me. I don't see any point in making heavy weather out of this very natural and very human bit of behaviour.