ACCEPT_LICENSE has support for things like "* -@EULA" which do not follow the usual incremental variable behavior. Since it has a different implementation, ACCEPT_LICENSE settings in profiles are not incremental with respect to each other, but ACCEPT_LICENSE settings in make.conf are incremental with respect to the profile setting.
Given the deviance in behavior from usual incremental variables, I recommend that profiles do something like this if they want incremental like behavior:
Thanks, this saved my day :)
PMS defines incremental variables in section 5.3.1:
"Other variables, except where they affect only package-manager-specific functionality (such as Portage’s FEATURES variable), must not be treated incrementally—later definitions shall completely override those in parent profiles."
So from a PMS point of view either behaviour would be fine.