This is a somewhat obscure bit of the <a href="http://www.w3.org/TR/rdf-syntax-grammar/
">RDF/XML syntax spec</a>, but after having looked through the grammar and text, we conclude that Rio and the W3C RDF Validator are correct in not accepting this syntax.
According to the official grammar, only <a href="http://www.w3.org/TR/rdf-syntax-grammar/#emptyPropertyElt
">empty property elements</a> are allowed to have property attributes (like rdfs:label). The host_of property in your document is not an empty element.
The sioc:link property, however, is an empty element, so in this case, using a property attribute (alongside a resource attribute) is allowed.
It seems to me that if it is true that raptor accepts this syntax, then this is a bug in raptor.