LTNman, I think you're right - the difference with Luton is that the Unitary Authority has a stake in it and felt it unwise for one of the operators to have a stake in it. Mind you the new terminal isn't the Taj Mahal which Stelios tried to portray it as before it was built. From the outside it looks like a tin shed, and the inside's not stunning.
Also Coventry isn't used by other airlines who may be upset by on of their rivals owning the infrastructure. Don't think Atlantic Airlines/Air Atlantique etc will be too worried about TUI owning it, but BHX and EMA may be!!