I hate to break it to you but well over 70% of New Balance shoes are produced in whole in China. Of the remaining shoes all but a handful are assembled in the USA but of foreign/imported materials. I have nothing against NB and acknowledge that they are creating jobs in the US in their assembly plants, but do not like it when people without knowledge put down the other brands for making their shoes abroad, because NB does it as well.
Moving on because that is simply off topic, lets get back to the point of this thread. The athletic footwear/apparel industry is a little different than most because for many of the employees their professional and social lives tend to blend together (this was mentioned previously). What I mean by this is many of the folks that work for major running brands are runners themselves, which means that when they go home at night and want to go for their run, they are in some ways bringing their work home with them.
There are a obvious reasons why a Brooks employee would not wear Mizuno to work, and ideally would not wear them at home for their run (even if before being hired it was "their shoe" and they are extremely superstitious. Yes, they could run in the Mizuno and if caught claim that they are testing the competition, but that will only work so many times.
Think about it this way, many of the footwear companies are publicly traded so the employees may be compensated in company stock, or have participated in corporate stock purchase programs. If you own Nike stock because you work for them, why would you contribute to sales of a rival brand, this is stupid on two ends, you are a. paying for a product you could get for free from your employer, and also not contributing to the potential growth in your company/one of your financial assets. There are other reasons, but to summarize, if you are working at a brand and you do not have the common sense to wear that brand to work, shame on you and shame on the brand for hiring someone as dumb as you.