Because it's 'family' money. When I worked all the money went into a joint pot and we spent it as we needed, irrespective of the fact that my dh has always earned about 3 times as much as I ever have.
When I had to give up work to move abroad to be with him, I felt guilty I wasn't earning, but he said it was saving money as we have cut costs by me not working. The food bill has dropped, I don't need work clothes, the petrol bill has halved and we are all happier as I am less stressed than when I was teaching.
I don't see why there is an issue about women being SAHMs, as it's a choice. I think it's appalling that there is now a culture where women are encouraged/driven back into the workplace, when they may prefer if they can afford it, to stay at home with their kids. The knock on effect of this policy can be seen in any classroom in term time, with kids turning up tired, hungry, no breakfast, hung over, with no social skills, and the reason is frequently, 'both my parents work, and I just put a ready meal in the microwave Miss and eat it in my room'.
If I was in the UK I would be working, but as I am abroad, I have stopped feeling bad about not working, as I gave up my career to move here; so the flip side is that my dh has to support me if this is where he wants me to be.
As was said earlier, how an individual spends their wages is down to them, so if they want to support their husband/wife/partner, what's it to anyone else? They've earned the money. However, it's when the benefits system is abused that people get annoyed. What really irritates me about it is that the Government lose billions per year in benefit fraud, yet if they had a joined up computer system, they could have sorted this years ago.