It might be an idea to check what the regulations are for specific aircraft types before accusing an airline of operating illegally. Different aircraft have different rules depending on configuration, density, design, etc.
I have operated the ATR 42 and due to weight and balance requirements, the self help exit at the forward was often left unoccupied as our passengers needed to be grouped together in the middle of the aircraft leaving the first few rows empty. We were never allowed to move the passengers, sometimes even after take off. Having also operated the E170, with the same weight and balance issues, it may well have been the case on this E190.
Although not having been there, I can't say for sure however you would think that if it was a requirement to have people sat at the exit, then the crew would have ensured their were briefed ABP sat there.