It seems to me that The Inn on the Lake and the Cathedral are both multiplicative tiles in that they multiply the value of every tile in the Road or City respectively, whereas pigs and pigherd tile(s) are additive features, as they merely 'add one' to the point value of each connected city.
But the fact that pigs and pig-herd tiles can be added together is only due to the fact that they're different—one's a meeple, the other a tile.
Also, I'm not convinced that the inn and the cathedral are multipliers. The cathedral adds a single point to every city segment (from 2 to 3), and so does the inn (although you could argue that it doubles the value from 1 to 2). AFAIK, the only strictly multiplicative factor in the game is the Cathar tile, which doubles the value for farms…