Even according to duo we are all butterflies and sharks and such and I'm guessing most of us work... ;)
I've known many people that could not function "normally" without their service animal working so hard. Just to help solidify the difference between working and functioning.
"Animal does not work" doesn't sound good in English, does it?
Djur is assumed to be plural here, even though the singular is also djur. Compare it to a similar sentence: "Hundar arbetar inte" (Dogs do not work). We would never say "Hund arbetar inte", instead we would specify which dog like this: "Den här hunden arbetar inte" (This dog doesn't work) or "Min hund arbetar inte" (My dog doesn't work) or something similar.
My point is: The meaning of djur here is animals, not animal because of how this sentence works with other words.
I'm a bit confused about the pronunciation. In the phrase it sound like the stress of the word "arbetar" is in the first syllabe, the "ar". But if you listen the audio help for the word by itself, it sounds like the stress is in "be". Is this correct? does it change when used in a sentence? or am I going crazy? :D
There are a lot of other comments, but I wanted to add another. . . . many animals work. And not just service dogs, but dogs that help herd cattle, bomb-sniffing dogs, horses that are still ridden and pull wagons. . . . I'm sure there are other examples, also. And, Duo, you work: you run a language school. . . .