The mystery of heating up canned beans- What the tin says
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.