Declating something "Kosher" or "Parve" is not a legal issue, anyone can do it. For that reason unless the product also bears the mark of one of the authorised Kashrut agencies, Jews who keep Kosher will not buy it. Putting the stamp of one of the Kashrut authorities on your product IS illegal as they are registered trademarks and using them without permission a violation of copyright and trademark law. Personally, I will not by anything without a kashrut stamp (except for fresh fruit and vegetables whih do not require a stamp) regardless of what claims the manufacturers make. Every Orthodox Jew I know does the same, only buying foods which have been certified kosher and not accepting the word of the manufacturer without accompnaying certification.
Orthodox Jew; Reverend