Because in the US (where I live) you are specifically asked to look at a list of activities that would preclude you from donating.
Two of them are as follows:
1) Most medications are ok, but you may be deferred if you are taking certain antibiotics, steroids, and other medications.
2) You cannot donate if you have ever injected drugs not prescribed by a doctor or had sex with someone who has.
So, it is not safe by any stretch of the imagination since you could be introducing any sort of pathogen into your blood stream each time you inject your gear.
As for number 2, I don't care what way you slice it, you're injecting drugs not prescribed by a doctor if you're using gear. Mentally you may think steroid use is OK, as do I, but others may not. Would you want blood from someone that was injecting heroin for 2 months? They may think heroin use is OK, used a good technique, and always used sterile needles. Even if I knew all of that, I'd pass. So even if you want to donate or lower RBC count and BP, you're being deceptive to donate blood by not being honest on screening questions. Just do some good old phlebotomy!

(kidding)