There are loads of walk-on fares that are TOC-specific! Any TOC that does not set the fare is allowed to do this. If they do set the fare, they can only do so for promotions, with some odd exceptions that no-one so far have been able to explain such as Milton Keynes to London, where LM have been allowed to set their own CDR fare, despite pricing the Any Permitted.
XC will get between a quarter to a third of the £10.30 Any Permitted fare, but will get all of the £8.10 fare (if bought on board - which they are happy to do), or about 90% of it if you buy elsewhere. So you can see why they do it. There would be nothing to stop East Coast doing the same, but EC don't generally sell EC-only walk-on fares except for in Scotland.
I am surprised you've not heard of this before, there are numerous threads about it. All the open access operators offer TOC specific fares, and many of the 'commuter' TOCs that share routes with 'InterCity' TOCs do so too. Plus many more.