WHBM,
Your exceedingly concise analysis does assume that the company can't in some way get its hands on a chunk of the pension fund, which is not always the case!
Interestingly, for all Lord Weinstock was vilified for sitting on cash mountains rather than investing, he apparently personally insisted that when the Stanhope Pension Trust was formed (which was responsible for GEC pensions), it was made a rule that not more than 5% of its assets could be invested in GEC or its subsidiaries, thus preventing the possibility of a Maxwell.
There is another argument here which says that a contract is not a contract if one party can change it unilaterally. In other words, how do you trust the BAA not to keep changing things, such as not even contributing to any pension scheme? Rather like the government on police pay - go to arbitration and then not accept the arbitrator's award.