It is not a small barrier. It is a warning indicator. If there are already some problems (minor) on the site, there may be also hidden problems with the product I wanted to buy. The quality of the site is just another indicator to guess the quality of the product I am buying.
But having a user register for an account isn't inherently a problem with the website, not like a error, missing page or broken feature. It is merely a step in the purchase process that needs to be optimized. I wouldn't assume that the requirement to register for an account (which is completely reasonable, although unnecessary)would indicate that there is a problem with a physical product they produce.
