This actually makes sense. What I assume is being said is that each individual member of a BMI HHA can transfer their miles to their own individual BA account (which may or may not be part of a BA HHA).
This is aligned with the current BMI HHA process, whereby a redemption is deducted pro-rata from the individual balances of each member. Whilst miles are totalled in the HHA, the system still records them against individual members.
And unless that individual BMI member has a BA account in their own name (be it part of a BA HHA or not), why should they be allowed to transfer the miles?