Agreeing to put people off totally will cost LNER a lot of money, and give their competitors a temporary advantage, because people will either choose to drive most of the way, book a coach or a flight, or decide that they don't want to go to London at all. It's their right to agree the message cross-industry, as they have done, if they wish to.
Most people who have an urgent need to travel will notice if they put Doncaster to London into a journey planner, or speak to a member of staff, that they can in fact book a flexible ticket and make arrangements to travel on East Midlands Railway. People are not daft either - if they need to drive to a station and they see this warning if they pick Doncaster, they might pick Chesterfield instead, will not see the warning, and thus will be considered from there as booking normally.
Most people who have an urgent need to travel will notice if they put Doncaster to London into a journey planner, or speak to a member of staff, that they can in fact book a flexible ticket and make arrangements to travel on East Midlands Railway. People are not daft either - if they need to drive to a station and they see this warning if they pick Doncaster, they might pick Chesterfield instead, will not see the warning, and thus will be considered from there as booking normally.