A plea bargain conveys an "or-else" ultimatum.
Plead guilty, we'll recommend life without parole or else we're going for the death penalty.
With this proposal, what's the alternative threat ?? We'll deport you, throw you in jail, prosecute you ??
With this "plea bargain," as you characterize it, there is no threat.
The proposal is, please register or we'll do what we've been doing - nothing.
Or, in truth, instead of a plea bargain - its legal sanctuary with a reward.
Please register, and we'll reward you with a three year work permit and social security eligibility.
Instead of a carrot or a stick methodology, this is a carrot or a carrot approach, there is no stick to back it up.
"Please register, and we'll reward you with a three year work permit and social security eligibility. Instead of a carrot or a stick methodology, this is a carrot or a carrot approach, there is no stick to back it up." 
Indeed. If having the threat of being deported by a surprise INS raid hanging over your head like a sword of Damocles isn't a stick, or if being unable to get a credit card or a home loan without Registering isn't a stick, then this is indeed a carrot or not approach. Ditto if being blackmailed re: turning you in for deportation if you don't accept lower wages or pay a bribe.
Goodness! Why would any illegal alien want to dispel such innocuous concerns?! They should clearly choose to not Register for Bush's plea bargain.
< /SARCASM >