Tax on bloggers?

Click on this link to read a Washington Post story (8-24-10) about how Philadelphia is taxing bloggers who make money from advertisements running on their blog page.  They regard such bloggers as "running a business" and therefore expect a $300 "privilege fee" for doing so.  Do you think this is a legitimate form of taxation?  The article lists several other web-based, advertisement-laden activities; should these be included too?  How long do you think it will be before the Web becomes the focus of attempts by government to regulate and tax it?