It's because FourFiveSeconds and BBHMM aren't very pop sounding.
It was the same when Diamonds was released, she got lots of backlash because it wasn't a dance song (and I remember it was released at the same time as Die Young by Ke$ha, and everybody at first was saying stuff like Rihanna should've released Die Young or that Ke$ha's era would go better since she lead with a dance-pop song).
Because she's always been seen (more than even most pop girls) as a manufactured pop star with no creative involvement who buys her hits. She also upset a lot of people with the way she forgave Chris Brown, took him back and recorded a sexually violent song with him. The only thing that couldn't be dragged was her singles success, and now that she doesn't even have that anymore, it's open season.
