Cleanse+Heal equality
If Rengar's gonna get a heal that's also a cleanse and deals damage in an AoE and also grants a second of CC immunity (even though CC is the only counter to assassins and mages are generally given only one CC ability so as to be balanced against assassins), then Gangplank's should also give him a second of immunity.
Or you know, the obviously much more balanced choice of NOT giving an assassin a costless cleanse.