If you actually read it there is an explanation. The EU budget for the cohesion fund to develop the infrastructure of 'deprived regions' was agreed years before we even thought about a leaving referendum . That budget runs until 2020 and we are legally committed to paying our previously agreed contribution.
All this was known before we thought about leaving the EU, it is NOT New!
It is just as if you buy something on credit and then want to end the arrangement early, you have to pay what was agreed before you can get out of the arrangement.