Scottish Airports used to do this as well and it caused no end of grief locally but there was a strategic rationale in managing the limited demand. PIK was long haul and GLA was short haul business and holiday traffic leaving EDI with limited business and very limited holiday flights. The group funneled traffic where they thought best and invested accordingly, when that strategy was challenged in court, GLA got long haul and major investment and PIK was sold off. However the eventual winner in the long run was EDI, but only when it was decoupled from the wider group and sold off to be a direct competitor to GLA. The group had, albeit with understandable intentions, constrained demand at EDI to boost GLA. Once they became effective competitors, traffic which had been growing at EDI exploded. What's in GLA's interests wasn't in EDI's.
There's something similar going in with MAG IMHO.