Why must you include the client on a delicate internal issue when it is involved with SMP?

Since Client is the representative of the stakeholder, all decision making will be decided by the client.

because the client is stakeholder

Handling internal project operations as well as managing a client relationship, project manager will find the line between clients and internal teams.To balance the line between defending your team and the project, and making your clients is bought between to make them understand the the issue related to which party.