Jade Wang from Google announced in the Google Business Help forums that Google My Business Locations now have the ability for businesses to set their primary business category.
Jade wrote in the forum thread:
Quick update -- you can now set a primary category for your locations if you are using Google My Business Locations. Please note that all locations of a business must share the one category that best represents the business. If you have multiple types of locations (e.g. sub-brands, departments, etc), this rule only applies within each of these sub-groups. Please visit our Help Center for more information. Thanks!
Here is a screen shot of where you set your primary category after logging into the Google My Business Center and selecting your business and editing your category:
For more details on this change, see the help article.
Forum discussion at Google Business Help.