I suppose the rationale for the kind of practice Geoff mentions there is that there's a chicken and egg situation involved. Until people can be persuaded to sort out their rubbish, it's not really possible to deal with sorted-out rubbish in an appropriate way; if they can get that to happen it'd be relatively easy to organise arrangements to deal sensibly with the pre-sorted rubbish.
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What tends to get left on one side is the fact that much/most of the stuff we chuck out shouldn't have been there in the first place - I mean all the extra packing that is such a nuisance to customers in any case.

There should be some way of putting the cost for dealing with this on the people who produce and sell the stuff. If they found it was more profitable to do away with wasteful packaging we'd all be better off.

The "free market" is supposed to sort out that kind of thing, but it doesn't seem to work. As with many of the things it's supposed to sort out.