CiviCRM does not record processing fees for certain PayPalPro transactions

When processing PaypalPro IPNs, CiviCRM does not record the value of mc_gross (the processing fee) for transactions where &txn_type=web_accept. I have not been able to find any reason in code for this to be the case, as it does record mc_gross when &txn_type=express_checkout, which triggers, to all appearances, the same process.

Forum thread here: http://forum.civicrm.org/index.php/topic,26687.0.html