Conflict of interest
Whether BAA treat BA differently or not is a debatable point, but because BA do not have a substantial equity ownership of BAA / Ferrovial, it makes it harder to claim conflict of interest as oppoosed to simply giving your biggest customer a better deal.
From what I can tell, most private companies in the world give their big-spending customers better terms. Where discretion permits, do Fraport really not sometimes try to make things slightly easier for LH, or AdP for AF ? I can't think of any long-haul airlines which don't have some sort of loyalty scheme.
If just a couple of airlines (who happen to be amongst the bigger customers) have a significant equity stake as well - then in my opinion the likes of Monarch, Thomson and others would be justified in publicly complaining about conflict of interest rather than the milder "they take more interest in their biggest spending customers"