When it is collected from a crime scene, it is because this DNA is out in the public and the person who owns it has given up rights to it. It is effectively public domain.
When a sample is taken from a suspect, they are asked. It is not just taken as that would be illegal. However, if a suspect were to decline giving a DNA sample, they are pretty much admitting guilt by denial. It's like saying no to taking a breathalizer test as part of a field sobriety stop. If you say no, it's likely because you are drunk and don't want to incriminate yourself. If you're innocent, then you have nothing to hide and should just submit to the test.
Cops can't walk up to a suspected criminal, tackle him to the ground, and just swab his mouth for DNA. That's illegal.