I think spousal sponsorships are exempt from that only for medicals.
Though it says there is no minimum income to sponsor a spouse or dependent children, CIC does refuse spouses on grounds of "financial inadmissibility", if they are concerned they will end up on welfare. In such instances, they will ask the person being sponsored to demonstrate their ability to provide for their family, failing which the application may be denied. There is one member, "gango girl" who went through this but eventually won her appeal.
The undertaking signed by sponsor and sponsored person (IMM 1344), says the person being sponsored also agrees that they will do all they can be self-sufficient. I'll try to find the OP Manual that talks about this and will post it.