I did an Internet search on Beta Reader. Almost every result said Beta Readers addressed grammar, punctuation, and other editing functions. (?)
Again, I was quoting authors in author forums, rather than any definitions defined by editors looking for additional work.
Beta reader - Wikipedia
An alpha reader or beta reader (also spelled alphareader / betareader, or shortened to alpha / beta), also pre-reader or critiquer, is a non-professional reader who reads a written work, generally fiction, with the intent of looking over the material to find and improve elements such as grammar and spelling, as well as ...
Ok, one point for you. But seriously, why would an author use a non-professional reader correct his grammar before they submit it to a proofreader (other than the similarities with cleaning up before the maid arrives to ensure they have nothing to do).
Beta Reader Group Group - Goodreads.com
A place to connect writers with Beta readers. Sometimes writers get so involved in the plot they can't see the wood for the trees. Hang on a sec'--that's a cliche and it's better to remove it. A cliche is just one of the things to look out for while writing.
Clearly, the beta readers see their role as being distinct from correcting grammar and spelling mistakes! Why would anyone assume readers would know more about writing then someone whose entire career is dedicated to understanding the English Language and how it's used?
P.S. I would NEVER hire an editor to beta read for me. As the article I first referenced specifies, that's more the role of Alpha readers, who are typically either Content Editors or other authors who'll spot plot holes before you commit to the full story. Beta readers, by definition, are used once the book is finished editing and is ready for publication.