Not everyone runs Windows. If you use Mac OS X or GNU/Linux to run Blockland, you will not get viruses through port forwarding because both were designed with network and multi-user security as the highest priority.
Doesn't matter. What I said applies to all OSes, regardless of ability of security. It is possible to be remotely exploited with a port forwarded. The odds of it happening may be smaller on Mac OS X or any of the various Linux flavors, but it may still be possible for it to happen.
Does WINE or equivilant even run Blockland, by the way?
The fact people think their safe because their a Maccigarette makes them even more likely a target. There aren't as many Macs as there are Window PC's, so theres a smaller hacker userbase.
What's relevant is the amount of viruses/exploits written. Most of the virus writers go for the most commonly-used software in order to infect/exploit the most people.