Not true; there is a criminal penalty. If you refuse, at least in every state I've had a drivers license you are still convicted of a DUI. You don't just lose your license, you can be subject to fines/jail/etc.
There is a distinction here that I think you are missing.
In the cases where you agree to submit to a blood draw as a condition for receiving a driver's license, there is no *criminal* penalty for refusing that blood draw. You just automatically lose your license for that refusal.
Now, they can (and probably will) still procescute you for DUI, they just have to get a search warrant to get the blood draw to get the evidence from you. Just because you refuse to submit to a blood draw "voluntarily" does not mean you are *automatically* convicted of a DUI. They still have to present enough evidence to convict you of that criminal offense. The refusal to submit might be used as evidence, but it's not enough on it's own.
Here's an example:
Roger gets pulled over for suspected DUI. In his state he has agreed to a blood draw as a condition for receiving his license. He refuses the "voluntary" blood draw and, effectively, his license is suspended on the spot. He suffers that penalty no matter what. The cop has probabla cause, based on his driving behavior, the smell of alcohol on his breath and possibly even his refusal to submit to the "voluntary" search. The cop then gets a search warrant and takes him to the hospital for a blood draw. When the draw is completed, surprise, his blood alcohol limit is *below* the legal limit. The prosecutor then declines to prosecute, so he is not convicted of any criminal charges, but he still loses his license.
Btw, in case you think this is unlikely, all I can say is I worked in ER's for years and saw lots of people brought in for a blood draw after suspicion of DUI. The majority of them would come in over the legal limit, but every now and then someone would be below the limit, sometimes way below.