Cans of beans say "Heat for 3-4 min. DO NOT BOIL". Does anyone know why canned beans should not be boiled? Surely they were boiled when they were first cooked, so where's the problem?
Beans in the can are already well cooked--they are essentially pressure cooked as part of the canning process.
While only a speculation, it is highly likely that they are now fragile and bringing them to a full boil would mar their appearance--fewer whole beans--from the agitation.
There is nothing I am aware of from a safety aspect that would contraindicate boiling the beans again.
I think it is to do with the taste, it's like making a soup or something. Boiling it can ruining the flavour that is why when making things on the stove it can be left to simmer but all recipes tell you to turn the heat down. Also boiling it risks burning the beans, I always look on the instructions as advisory; they also always tell you to heat throughly even thought it is perfectly safe to eat a tin of cold beans.