Again, difference between can and want to.
Are you seriously suggesting that I should fund my business travel myself just in order that CS might send me communications?
Tickets (and flights/hotels unless e.g. Ryanair/Travelodge they won't sell via the aggregators at all, though last time I booked sleasyJet for a work trip they insisted on the agent doing it for me via easyjet.com) booked via means other than our corporate travel agency
will not be reimbursed. Flat "will not". The expenses processing company will reject them and other than going to the C-suite level (if you've not come across that term before as it's quite recent business-speak, I mean Chief Executive Officer or similar) local managers cannot override this.
I think you're overestimating its instance based on your own experience. Nowhere I have worked in the past 10 years has required the use of corporate travel agencies, and only one place even had one, which nobody really cared whether you used it or not.
It's true at most corporates., largely because they want aggregated data of travel booked, but some also have stuff like pre-approval built in. Smaller businesses tend to be more flexible. It's a bit annoying (mainly because I like seat selection) but it is what it is.
However, it's slightly irrelevant. The railway booking system is federated, and I should receive proper service whichever accredited site I use - that's like the whole point of the accreditation system.