Why 2 input bias currents are equal for OpAmp?
The basic error you made is in your understanding of bias currents: -

Bias currents are not something that YOUR input signal is responsible for so, the direction or polarity of input signals are irrelevant. Bias currents can be regarded as currents taken (or given) by the input stage of the OP-AMP and they will flow into or out of your input voltage sources irrespective of their polarity. They are self-generated within the op-amp and not to be seen as some current proportional to your input signal level.