I'd say it's almost impossible to not get sick during the winter time, especially if you get in contact with so many people during a tour, shaking hands, possibly sick people at M&G's etc.
I'm not a big fan of winter tours as well, you easily get sick yourself. Waiting outside sucks. Just like Mark said, summer tours are more fun, the mood is better and you have more energy yourself to do stuff.
I was sick during my 2 DBS shows in February 2010, I had fever and felt like a zombie. And almost every time I did several shows in a row, a road trip, I got sick afterwards haha. I got home from the US last year and almost needed an entire week to recover and got sick as well.
I don't blame them for canceling shows, especially when Chester is sick with even a cold it attacks his voice, we all know that feeling.