Copyleft is just a hack to route around copyright damage. Absent governments enforcing it, we'd all just either release code or not release code and the licensing friction would all go away.
GPL does far more than "route around copyright damage" - its aims are to give the _end user_ freedom, freedom which often wouldn't exist even without copyright.
Lets look at how things work with GPL'd code:
1. Developer A writes some code, releases it under the GPL.
2. Company B takes A's code, modifies it a bit, maybe integrates it into a product (mobile phone, TV, whatever), puts the finished product (i.e. including the binaries) up for sale.
3. End user C buys B's product, wants to modify it, so asks B for the source.
4. B gives C the source, since the GPL says they have to
5. C is happy since he can now modify the code.
(Ok, so it isn't alays plain sailing - B often has to be threatened before they will comply with the GPL; in the case of GPLv2 code, B's product may be Tivoised; frequently devices have a mix of GPL/proprietary code and its extremely difficult to integrate modifications into a device with only the GPL code, etc. but in theory at least this is how it should work).
Ok, so lets look at how this would work if there was no copyright law:
1. Developer A writes some code, releases it.
2. Company B takes A's code, modifies it a bit, maybe integrates it into a product (mobile phone, TV, whatever), puts the finished product (i.e. including the binaries) up for sale.
3. End user C buys B's product, wants to modify it, so asks B for the source.
4. B tells C to piss off because the source is a trade secret and B is under no obligation to release it.
5. C cries.
The GPL relies on copyright law to reach its goals of giving the end users the freedom to do as they wish with their own devices. Without copyright, the end users would be in a much worse position since manufacturers could use any freely released code in any way they see fit with no obligation to their end users at all.