This BBC is not funded by taxation. It's funded by a subscription paid by owners of TV reception equipment. Some people who pay the licence fee also pay taxes, but some don't.
> you can't use the platform like your own personal soapbox.
Lineker didn't express his opinion on the BBC's platform. He used his personal account on Twitter, which is clearly separate from the BBC.
Whether licence-fee payers expect BBC sports presenters to refrain from expressing political opinions outside of their work for the BBC is debatable - as far as I know, nobody has consulted them in a systematic way.