Every line of work has slang for it's customers [insert name of choice] and you can ask the 'caring' professions too. A medical doctor pal of mine has the most ghastly things to say about some of his patients - but when he is with them, he takes great care of them. That's what human beings do.
I have been in service industries all my life. From aged 16 when I worked in a department store on Saturdays and school holidays, through telecommunications, then a consultant in that field to now in a highly specialised field that involves great delicacy, tact and all sorts of stuff with people that emotionally highly vulnerable. Today, a family that I worked for on Wednesday emailed an apology for their demands and behaviour. Mostly they don't. Sometimes, when I get back from helping people - I have been known to say some unkind things about them ... welcome to the world!
SLF? I think it's a VERY funny label.