In file etc/auth/jmx.acl.org.apache.karaf.bundle.cfg there is an rbac rule:
setStartLevel(java.lang.String, int)[/([1-4])?[0-9]/,/.*/] = admin
from what I understand and what seems logical, setting start level below 50 to a bundle can do only admin. When I try to use this method to set start level to a number below 50 using a user with only manager privileges, the method is invoked successfully.