They're not so arrogant about their code being bulletproof that they're willing to hand it to an adversary and say, "Sure, knock yourself out - see if you can find any holes"? Yeah, I'm not sure that I see a problem here.
Yes. Handing out code to everyone (open source) can provide many benefits, but there's no benefit to handing it over only to agents you do not expect to cooperate (foreign governments, or governments in general).
Can understand the worry for malware products from startups and innovative companies that are demonstrating an impressive work and surely need that secrecy to thrive against the market gorillas. Symantec can't really be called innovative since 2009 or so, doesn't make much sense the security by obscurity mantra unless there are other reasons.