i've noticed that as well that's actually what confuses me the most if it really is a single why can't i buy it anywhere honestly i think that if people write it as a single they don't really care if it isn't available separately simply calling something a single doesn't specify whether it's promo or not for people trying to cater to consumers like billboard com reviews if it's a popular hit and or music video people know it it doesn't matter under what technical details it was released but i think the difference is that since we are able to specify this we probably should we should try to reflect the reality i don't think having a music video and a high placement in charts does anything to change the type of single it is i've seen music videos for promo singles many times before even some for album tracks isn't it a promo single nevertheless if it's only available separate from the album for promotional purposes it's not like this is degrading the single in any way either some artists just don't release subsequent album singles separately for consumers because they'd rather prefer they buy the album