The main issue with this is that sales of hot drinks are very lucrative; the markup is massive and they are one of the best selling items. Indeed I'd hazard a guess that it quite possibly underpins the catering service in some cases. Giving it away for free by no means guarantees that people will buy something to go with it (unless of course you made that the deal; buy something and get a coffee etc free).
There's also the logistics of it. If a trolley is making its way through a lengthy train (let's say a nine car IET), offering a free drink to everyone on board, it's going to take an age to get through the whole train, and miss a lot of potential sales. Also consider that a catering trolley has a surprisingly limited supply of hot water - a Hitachi-provided tea wagon on an IET will do about 45 cups before it needs to go back to the kitchen/trolley point and replenish the urn. You can manage with that if you're serving a few people in each coach, but not if you're doing freebies and they all want it. It just wouldn't work.
