Because to a non-libertarian, anything they consider good is a right and must be mandated by force if necessary and anything they consider bad is pure evil and must be outlawed by force if necessary. There is no grey area between must-do and must-not-do. "I'd likely do it, but I wouldn't force someone else to." is an incomprehensible position.
To be completely honest, I consider forcing someone to do 'good' (or punishing him for not doing 'good') evil.
I'll just move over rkbabang's strawman and go straight to the more interesting point: Situations aren't binary, and evils aren't all on the same level.
Not curing your sick child when you can is evil, and maybe someone forcing you to cure your sick child is also on some level evil, but are they equivalent? And is the good that comes from your child being cured more than balancing out whatever "evil" comes from you being forced to do something that you wouldn't do voluntarily? In the real world, there are many variables that need to be balanced out and taken into account, as inelegant as that makes the model to people who like to see everything in black and white.