So why is it a disfellowshipping offense to take a blood transfusion when there is no precedent in the Bible for it?
Taking blood is not a direct disfellowshipping offense.
Following are the two policies:
1. If someone takes blood willingly and is not repentant, he is termed as disassociated.
2. If someone takes blood under extreme pressure, he is given counsel.
when there is no precedent in the Bible for it?
In JW world, the precedent for the above policies is given in the two instances you mentioned:
Remember that when someone was observed breaking the Sabbath, they reported it to Moses and they didn't do anything right away. But then Jehovah said to put him to death.
In this instance, it is taken that the person willfully broke the Sabbath law and was punished with death. This is the basis for disassociating a person who wilfully takes blood and does not repent. (Policy No, 1 above)
when it was reported to Saul, Jehovah did not say anything to them, no death, no punishment, just that they should not have done so, and he provided to them a slaughter.
In this instance, Saul repented and provided special sacrifices which were accepted by Jehovah and was pardoned. This is the basis for not disfellowshipping a person who is repentant after taking blood due to extreme pressure.
The above is the JW policies.
However, whether blood transfusions fits the Biblical law is a different discussion.