It depends on many factors, not all of which are sinister.

For example, the board officer may have been previously authorized to negotiate in the name of the co-op, and given prior authorization to enter into a contract up to a certain dollar amount. This is done to save the rest of the board time and effort where their contributions would be of little value.

Yes, I would want such a duly authorized board member to report back to the rest of the board on any binding actions s/he took, but when a single board member is authorized to negotiate for the board, it implies a level of trust and acknowledgment of their effectiveness in the negotiations.

Each board has a different way of dealing with issues that come before them and they have to deal with. Without additional information about the nature of the board officer's dealings with the contractors and vendors, it's difficult to make any judgments.