Standard Errant: a standard doing likewise?
1) The RDF model is roughly equivalent to a relational database model where all tables a restricted to two columns. The model has several quirks caused by early infection with XML (curse you, language tags). RDF graphs are simply sets of triples, and the metaphor of graphs is not essential to the interpretation.
2) Merging two RDF graphs may not be zero-cost, formally, because blank nodes. But more importantly, merging two RDF graphs may not be zero-cost informally, because they may use different vocabularies, or the vocabularies may have been improperly applied.
Because of the choice of logics used for the standardized semantic web, the results of merging a second graph aren't allowed to override a conclusion derivable from the first graph.