Summary

A customer (Jira Service Desk) can be added as a watcher to a jira-software issue.

Steps to Reproduce

1. Create Service Desk Project and JIRA Software Project.
2. Invite or Create a Service Desk Customer i.e "usera" (user without any permissions or application access) to JSD project.
3. Login as a user with the manage watchers permission i.e. "userb" on a JIRA software project.
4. Create JIRA Service Desk issue "TESTA" and JIRA Software issue "TESTB".
5. Add customer "usera" as a watcher to the JIRA Service Desk issue i.e "TESTA".
6. Add customer "usera" as a watcher to JIRA Software issue i.e. TESTB.

Expected Results

Test A.) Adding a customer "usera "as watcher to a Jira Service Desk issue "TESTA" would not be allowed as it only applies to jira-users with the correct permission not customers. The customer should be added as a Request Participant to the JIRA Service Desk issue instead.
Test B) Adding a JIRA Service Desk customer "usera" as watcher with (no application access or Jira permissions to the JIRA software project/issue) should not be allowed.

Actual Results

Test A.) Adding customer "usera" as a watcher to JSD issue "TESTA") results in a permissions issue "The user "usera" does not have permission to view this issue. This user will not be added to the watch list."
Test B.) Customer is added as a watcher to the jira-software issue (with no jira-software or permission access to the project/issue) and can receive issue updates.

Notes

I was able to reproduce this in my test instance JIRA v7.1.0-OD-06-005 with JSD Version: 3.2.0-OD-01-002-D20160204T062648 and JIRA Server 7.0.10 and JSD v3.0.10.