Because, so far as the subsidiary is concerned, that interest is a true finance cost.
What you are apparently confusing is the ignoring of intra-group interest paid / received for consolidation purposes.
Adjustments are NOT put through against the respective PorLs. The ignoring of these amounts is a presentation point only – it would be stupid to add as a finance charge AND as a finance income an amount payable within the group. But each separate company’s PorL shows the correct position. Only on consolidation do we ignore these amounts