Professionals have special needs and abilities to price less competitively than commercial organizations supplying goods, and special responsibilities follow from them not to exploit the client. This is the moral case for professional responsibility.
There is also a business case, which is that costs and risks are reduced by being professionally responsible.
The PMI has a professional code of conduct, to which PMPs must sign their agreement.
The PMI's code is split into responsibilities to the project management profession and responsibilities to customers and the general public.