I can’t see why it should be against the rules. Company B is not worthless in the end - after selling its shares in C to A it keeps the money, and the price is fixed. Of course: If B’s CEO decides to do nothing with the money you won’t get any return and nobody will buy your shares. But that is a risk you take with every investment in AS, it’s not specific to this situation.
The root of this is that a subsidiary of a publicly traded company doesn’t need to pay dividends. There’s an easy way to establish an airline with extra money but no obligation to its shareholders:
Operate an airline2) Do an IPO3) Sell/return all planes4) Use all money to found a new subsidiary