Why would this be surprising? It's a buffer overflow in a program with root access.

Is there something about Macs that prevents buffer overflows?

In particular: if I understand correctly, the antivirus software has code that decompresses stuff, so that attackers can't hide bad stuff simply by compressing it. The flaw was in the decompression code. I would expect that code to be shared across platforms - there's nothing Mac-specific or Windows-specific in how you decompress a byte stream.

Why is it surprising that it affects Mac?