They are just lean. As you said, they are in pasture condition. They are a long way from being skinny and they are a little out of tone. You have nothing to worry about with the Humane Society.
That woman was just being ignorant. Probably one of those PETA fanatics. /
They look healthy even if they are not fat. As for the last pic of the red mare, I think her look has less to do with being skinny than it just being her conformation.