Because obnoxious people will spam the form with emails that are not their own, as malicious behavior against acquaintences they detest. These ubiquitous subsc

Because obnoxious people will spam the form with emails that are not their own, as malicious behavior against acquaintences they detest. These ubiquitous subscribe forms provide a vector for inflating the size of mailing lists. Mailing lists with more addresses sell for real money, and sell well. It doesn't matter if the data is accurate. Only the perception of the data matters. The more addresses the better, the more datapoints per address, the higher the quality of data. The age and activity against the data matters. There are reputable companies that trade on these details and pay the salaries of expensive developers.