how to understand the capacity in binary symetric channel (BSC)
Think about the extreme. If your bit error rate is 1, then you know with certainty that the bit was flipped. If you receive a 1, the value sent was 0. So, flip the bit. This is why p=1 is the same as p=0.

Similarly, any time it's more than 50% likely that the received bit has flipped, you get better ber by just flipping the bit. Does that make more sense now?