As is pointed out in the XML spec, DTDs aren't just for validation; they also tell you what the default value of attributes are, and potentially contain entity declarations. In a way, DTDs contain some of the document's information. So, because these things are needed to 'read' the document, conforming parsers are required to read them if they are declared -- whether or not the parser is validating.
If you're sure the document's valid, you could strip out the DOCTYPE declaration before siccing your parser on it.
update : an even better solution would be to get a local copy of the DTD and use catalog resolution to find it. A catalog resolver can tell a parser where to look for a DTD with a given public ID, irrespective of the system ID (the URL in the doctype declaration).
If not P, what? Q maybe?