The game's designed to have every class on one character. What's the point in that if you need an alt to be able to rerun content. The design of the class system encourages one way and the design of the lockouts encourages the other.

As such, in my opinion based on my observations, I can conclude that SE never intended for us to Level alts to rerun BCB and they didn't consider that as part of the equation when they decided to put the lockout on BCB in the first place. What I would like is for them to take a stance on this issue, e.g.

Either:
A: We are aware alts can rerun BCB when their mains have done it already and this is working as intended.
or
B: We are aware of this issue, but have not yet decided how to handle it.
or
C: We are aware of the issue and will make the lockout account wide.
or
D: We are aware of the issue and we will be adding a loot lockout system to BCB to alleviate the need to do this.
etc etc etc.