The CC by-sa license requires derived works to be free as well. If someone wants to include my image in a non-free product, she still has to pay me to use it.
The equation commercial=evil doesn't work here, because commercially added value must be free.
When posting comments on blogs etc., I frequently use nonsense eMail addresses like email@example.com or firstname.lastname@example.org. Don't blame me for the spam they'll get. How shold I have known that they'd become real addresses some day?
Why won't sharks eat lawyers? Professional courtesy.