Beamer is a slang term that has been around for years. People don't know any better.
Here's the story: in the 1950's BMW raced motorcycles against a company called BSA. BSA motorcycles were nicknamed "Beazers". As a result people started calling the BMW bikes "Beamers".
The term Beamers applies to just the motorcycles. The term Bimmer applies to the cars only. Beamers has been around for so long that people don't know, or don't care to know the difference. Some people look at you stupid when you try to tell them their is a difference. I usually correct most people unless I can tell theirs not going to be a point.