His painting shredder piece suggests that he's specifically making the point that his art is temporary and that ties in with his general graffiti style. If he wanted it to be easy for people to buy and sell his art, he'd do it on something a bit more portable than the side of a house. A lot of his art around Bristol (where I live and Banksy's home town) is very much designed for the public to enjoy rather than to be traded.
As I read it, I don't think anyone from the city council forced them to do anything, "could put a preservation order on the piece" doesn't mean they did.
It's all a bit unclear though what the exact situation is though and why they chose to spend £200,000 instead of £200 on some paint.
Given the later "it's their property and right to do with as they want" comment, I think something along those lines is more likely, but hard to be sure.
The reporting on all of this has not been of stellar quality.
It says: >But after the Coutts’ were told by Suffolk council it would cost tens of thousands of pounds to maintain every year - they made the decision to have it removed.
Mr Coutts told The Times: “At first it was obviously incredible but as things have gone on it has become extremely stressful. I’m not sure Banksy realises the unintended consequences on homeowners. If we could turn back the clock, we would.
“We had the council get in touch to tell us that they could put a preservation order on the piece and we would be responsible for maintaining it at a cost of £40,000 a year.