The Welsh Government that owns Cardiff Airport formally objected to Bristol Airport's expansion planning application which was rejected last year by North Somerset Council (the rejection decision is currently subject of appeal). In the mid-1990s when BAE sought permission to set up a city airport at Filton one of the objectors to the plan was Bristol Airport. The secretary of state rejected the BAE application after a planning enquiry.
It's about businesses trying to protect their own interests. There is nothing illegal or immoral about it.