You could just as validly say the same thing about pretty much all mass-market software. Replace "copyleft license" with "terms of service" or "end user license agreement".
It's hard to think of any mass-market software that would not be covered under that interpretation, which is a strong hint that the interpretation cannot be right.
What I believe (3a) is trying to cover cases where a Party (e.g., Australia) is acquiring software for its own use. For instance, suppose Australia wanted to use AgileBits' 1Password [1] on all the government's desktop computers.
1Password is mass-market software, and this treaty would prohibit Australia from requiring that AgileBits make the source available as a condition of selling to consumers in Australia.
But if Australia negotiates a deal with AgileBits to supply Australia's government istself with 1Password, rather than the government simply buying it the same way consumers would, (3a) applies and the treaty does not apply.
[1] Picked for this example because it is the only well-known mass market software that I could think of offhand that (1) a government might reasonably want to mass deploy internally, and (2) is owned by a company that is in a country (Canada) that will be a Party to this treaty.
More generally, it would seem very out of form for a free-trade deal to include provisions that undermine the freedom of contracts. Contracts are an important part of capitalism, after all.