This looks like it was badly thought out. Funds in transit from buyer to the Bill.com account should be in an escrow account. Most escrow accounts are not placed where they gather interest as the time in escrow is small and rates are low. If interest is gathered = not Bill.com's as it is not on their $$. These accounts are usually run by banks who pay no interest but the $$ enters their churn and earns the bank interest - banks run these escrow accounts as a quid pro quo for the interest. It is possible that bill.com wants to do this on the same terms as arm's length banks do it - also keeping the interest. On a machine system this movement and tracking of who owns what is far cheaper than in the days of green eye shaded account managers with adding machines = a possible substantial aggregation without much costs as their system already does the lion's share of these transactions? Nothing wrong, these small increments are not significant to each account(some are larger though?) and as long as retention on the way to the client is small = the aggregate is also small, but being an aggregate could be valuable in aggregate and earn a little for Bill.com.
If all that was clear, most would let it go as Bill.com's service is useful and economical and it usually a lot cheaper than the traditional thieves.
So Bill.com - Truth up, we like you any way even if you earn a few bice on this float!