This is the rule of every prominent style guide that is professionally used. The rule is rather simple with little variation: All proper nouns must contain at least one capital letter at or near the beginning of the name, but it also must not be excessive or look ridiculous (ie, LONGALLCAPS and CaMelCaSe are ridiculous). AP goes a step further and also capitalizes the first letter in iPhone if it starts a sentence, as in IPhone.
If a brand name like iPhone insisted instead to be just iphone, all lowercase, then they would automatically be referred to as Iphone in regular print to recognize the proper noun, whether at the beginning or middle of a sentence or in a list. Basically overruling the wishes of the marketing droids.