"decided", huh? Like some have "decided" that they cannot carry weapons with them because legislation forbids it, I suppose?
Yes, "decided". All of those people "decided" to sign that document, unless you have proof they were "forced" to sign it.
You make a "decision" to get out of bed every day. You make a "decision" to put on clothes every day.
If you drive, you make a "decision" to follow the rules of the road.
As I stated previously, your "decisions" have consequences. Even "indecision" sometimes has consequences.
The OP, or anyone else for that matter, makes a free-will "decision" to live by the law of the land or not, and the actions that they take as a result of their "decisions" will, as I said, have consequences.
If the OP "decides to sign the selective service forms, that simply means he/she is registered for service. It does not guarantee a "call to arms", nor does his/her "decision NOT to sign, guarantee a summons to court.