Applying for industry job after accepting postdoc offer It is clear that once one accepts an academic job, it is just not right to continue interviewing for the same type of academic job. 

However I (and most others on both sides, I would imagine) think it is perfectly reasonable to continue applying and interviewing for permanent faculty jobs after accepting, say, a postdoc offer. Call this situation A.

However it is much less clear to me how the situation can be viewed from the new employers' perspective if after accepting an academic job --- whether postdoc or faculty --- the future employee applies for a (dream) industry job that just opened up. Call this situation B.

I fail to see how situation A is fundamentally different from situation B, unless I miss something.
After accepting a postdoc position, it is not necessarily appropriate to keep seeking a faculty position.  This might be appropriate if the faculty position was expected to start far in the future.  The fundamental difference between a faculty position and an industry position is that industry positions are usually intended to be filled in about a month, while faculty positions are filled after somewhere between a few months and a year.  This allows the faculty position to be accepted without conflict with the postdoc position.