Looks like timing was the issue with the companies listed, Many times ideas are sound, execution is solid but the audience just isn't ready for the actual product in the current landscape.
Shipping 50lb bags of dog food at pets.com for free is still a bad idea.
"Shipping for free" is a slippery concept, since it just means that the cost of shipping has been moved out of a line item called "shipping" into another one. But there are certainly people making money today shipping big bags of dog food for not much money. Amazon alone is full of them, to say nothing of these new retailers starting up. (I'm curious what they expect to offer me over Amazon, but that's a question for another article, I suppose.) It doesn't seem to me it was a "bad idea" so much as "one whose time had not come yet". Pets.com couldn't afford to set up Amazon-scale infrastructure for mere pet food, but now, Amazon-scale infrastructure exists. Even if these new businesses go under, the idea is frankly already proved to some extent, since people are already doing it, and I wouldn't expect them to stop.
Amazon also is clearly no making profit doing what they are doing either. They can afford it due to the vast scale but for how long?
Seems to work for me, not from pets.com specifically though. We have all our pet food on automatic ship.