Thank you very very much for posting this!
You have no idea how much this issue has distressed me. I had to come up with a work around to get past this limitation as I could not understand why this restriction was in place.
However, may I suggest that this be offered as an option instead of a default.
If you look at V11, it comes with an option under the account settings :
‘Allow Cost Center In Entry of Balance Sheet Account’
The same would be applicable in this case where the user would choose this option as you have done where it would be
Allow Project In Entry of Balance Sheet Account
That way, those people who have operated successfully over the many years without having the project entry in their balance sheet accounts will not be surprised as doing this action by default will result in many accountants being not too happy