They are striking because London Underground regard Boxing Day as a normal working day, which most people would agree is not a normal working day.
It would be interesting to see the agreement that LU are talking about, that ASLEF had previously signed up to which covers Bank Holiday cover.
I think they recognise it as a Bank Holiday but pay as a normal as part of an agreed annual salary. I.e. Some Food Manufacturing contracts, as you are required to manufacture 365 days per year contracts are worked out and paid based on a shift regardless on what day your shift day falls, the BH pay is built into the overall hourly rate so they can't have it twice!