why cant you donate blood right after going to the dentist?

The mouth and gums have an extensive mucosal blood supply. During moderate or complex dental procedures, there is the possibility of transient bacteremia from normal bacteria found around the teeth and gums. While this is of no concern to a healthy donor, if oral bacteria gets into the collection bag, there could be growth. However, the small amount of contamination would usually be handled by white blood cells in the donated whole blood. The FDA does not make a specific recommendation on post-dental procedure blood donation, suggesting they do not consider this a risk for bacterial contamination. Most collection facilities do not ask about dental procedures, but each blood center must determine their own eligibility guidelines. In general, simple cleaning, cavity repair and/or cap placement should not require a short deferral. Contact your regional center for specific guidelines.

The most likely reason I see is most people that have dental work have some blood in thier mouth from cleaning/dental work and they don't want possible pathogens in the blood that get their from the mouth.