The idea that people can continue to be paid a full salary when the demand for services (not just transport) is falling across the economy is fanciful.
Unless I miss my mark they may not have a choice. As far as I'm aware if you're contracted to work, say, 35 hours per week then your employer pretty much has to either give you 35 hours of work or pay you for 35 hours of work. I am, of course, speaking in broad terms. If someone's contract makes provision for being laid off for a period of time or being given reduced hours for a period of time then those provisions could be used. Equally if you're on zero hours then clearly the employer doesn't have to give you work then either!
Sticking to the transport sector though I it was interesting that BA didn't tell it's staff "you will be taking unpaid leave" it asked them to take unpaid leave. I suspect that that's because BA don't have the choice as I rather suspect their staff contracts don't have provisions for them to force them to be laid off.
Obviously many contracts have variation clauses in them that allow an employer to change the contracted hours of work but that can take a bit of time to work through and in unionised industries like the railway or airlines isn't exactly a risk free option. Of course in an extreme you can obviously make staff redundant but that does then make it hard to ramp back up again on the other side of a temporary slow down as well as the industrial relations issues...