Mangaka switch it around sometimes to give it some variety. Though I've read several of hers and they were almost all like this. Anyway, some people like to see things switched up every now and again.
I love all kinds yaois that tell incredible stories but this kind of story like this one, which is also twisted as seedy, always put me quite uncomfortable.
The only probleme I have with this story is that the uke is supposed to be the seme (according to yaoi's rules !) ^3^
Mangaka switch it around sometimes to give it some variety. Though I've read several of hers and they were almost all like this. Anyway, some people like to see things switched up every now and again.
I can understand that it may please to readers. It takes us out of our comfort zone so I guess it's true that it can be interesting to discover.