You were not forced at all to sign the plea deal. You had the option to do that, or go to court, or even become an informant for the detectives (was turned down though since they said you wouldn't be able to provide much).
For me to force you do to that would be something like "If you don't sign this, I'm gonna kill you/add more charges" or prolly a "/me puts a pen in the suspect's hand and forces him to sign the plea deal given to him" (This would be powergame)