> A HM Courts and Tribunals Service (HMCTS) spokesperson said: "The Royal Courts of Justice is a listed building and HMCTS are obliged to maintain its original character."
This is the why. HMCTS don't have the option to "just leave it" because they're obligated by legislation to remove the graffiti. Discussions about whether it'll bring in tourism are irrelevant to the discussion. The law requires the listed building to be maintained in its original state, so the government department follows the law.
As for the costs:
> Court staff barricaded off the area and guarded it. Attempts to clean off the mural began immediately.
> Westminster City Council will have to inspect the wall and if the removal is deemed incomplete, another cleaning method will have to be used.
> As a final option, the affected stonework may have to be removed and replaced.
> A Listed Building Consent (LBC) application has been submitted for the installation of railings across the wall that Banksy targeted.
All of this costs money, it's not just a case of putting a bit of turps on the wall and job done. It'll require experts to remove it without causing further damage.