Opportunism might be seen as a good thing in business because it can mean more profits. But it is not a good thing if two companies, for example, are partnering for a project. In that case, there has to be trust between them. To me, that means neither party will act as opportunists and will think of the other company's welfare as well.
If there is no partnership, and companies are in competition for business, it's a different story. Then, it is expected for them to take advantage of every business opportunity to increase their profits, even if they are stealing others'.
It seems a bit cruel, but I think this is what the economy is based on. The strong grows and the weak goes out of business. If a company is not opportunistic, it might not last too long in a sector.