Companion planting is a method of deterring pests by planting specific herbs and flowers nearby vegetables to deter specific pests. These herbs contain oils or scents that deter insects from the surrounding area. Sage is one of many herbs useful to this end.
Sage can be used to deter more than just bean beetles. It is also good at deterring carrot flies, flea beetles, cabbage moths and slugs. It can also be used alongside plants other than beans with positive results, including rosemary, cabbage and carrots.
Though sage has been cited as an effective companion plant for the prevention of bean beetles on bean plants, other herbs are equally effective, most notably rosemary and savory.
If beetles are found on a sage plant, it is extremely unlikely that they are bean beetles since sage deters bean beetles. The beetles are more than likely ladybugs, which bear a strong resemblance to bean beetles but are actually beneficial predators that are good for your garden.