I'm sure that, at the very least, the homeless person gets a bill even if there's a realization that he won't be paying. Of course a big difference between the police and a homeless guy is that the police have money. Similarly, EMS has a duty to respond to emergencies. Hopefully a "help, we need a blood draw" call is put on the lowest possible priority level since it isn't, by it's very nature, not an emergency. After all, if they really need a blood draw, why not transport to a medical facility or have their medical team at intake draw the blood?
Well... sometimes there are situations where you need a blood draw on someone, but you dont have enough probable cause to effect an arrest for DUI of either alcohol or other drugs, so you do the blood draw, release the person on scene, and once the blood test comes back, obtain the arrest warrant and go pick the suspect up.
Taking a person to a hospital like that or to the jail for a blood draw sometimes isnt practical, and it creates an interesting custody issue... if they dont want to go, and you dont have enough PC to make an arrest at the time, how can you justify detaining someone and taking them to the ER or jail? You cant. Its easier to call someone to the scene and do the draw.
Our agency only sends a bill if we transport or administer ALS treatment on scene (ie: diabetic hypoglycemia) so there is no charge for us to do a blood draw for PD.
The most common scenario is that we get called for a car accident, arrive on scene, find that there are 2 vehicles, one is the victim who is deceased on scene, and the suspect that is either being placed under arrest for DUI, or some other charge, and the officer needs a blood draw, the sooner the better to preserve evidence, seeing as how some intoxicants can be out of the system by the time you do the draw at the jail or ER. The suspect is not injured and does not need EMS transport, and either the patient refuses or the officer refuses for the patient. We dont transport, but we are on scene doing paperwork on the deceased, and do the draw in the ambulance for the officer.
The other scerario is that the suspect is not injured, and at the time of the accident the officer doesnt have enough of a PC to make the arrest, and they do the blood draw as more of a procedural event, to aid in the investigation and either clear the suspect or implicate them. We do the paper work on the deceased, do the blood draw on the suspect, and obtain a refusal from them.
Either way, we would have already been there so its not really taking a unit out of service just to do the blood draw.. its just another thing to do while we are on scene before we clear. Plus, if there is a deceased, we're just sitting there waiting for the ME to come take custody of the body anyway.