It would only be fraud if the person sponsoring multiple immigrants lied about it on the forms, no? One person can sponsor multiple (unrelated) immigrants, but as I recall the forms ask have you ever sponsored another immigrant before, in which case you must answer yes and give their information.
I dwell on this point because I am surprised people rush to the conclusion that this practice is illegal or immoral in some way. The I-864 is essentially a way for the US government to protect itself from financial liability by making the petitioner (and joint sponsor) responsible in case the immigrant becomes financially dependent on the government. If someone wants to sign such a form and take this responsibility, who is to say it is wrong?