I've never heard "Buddhism is selfish" before, but I suspect this is a side effect of Buddhism being severed from its cultural roots and being repackaged by the west as an "exotic" new-agey, self-improvement program. Buddhists in the east takes Buddhism's world view seriously -- all beings share a common fate; what you do to another being, be they humans, animals, or even inanimate objects, will affect yourself in due course. People who take this view seriously will practice an enlightened form of worldly concern, to be keenly aware what the consequence of their actions will be. To reach that stage they must seek to understand other people, and that is not what a "selfish" person can afford to do.
Buddhist folklore is full of stories of true altruism (such as
Prince Sattva sacrificing himself so that a tiger and her cubs can feed). While several Bodhisattvas having achieved Nirvana themselves and hence no longer have to endure the throes of reincarnation, nevertheless consent to be reborn in order to save people. The most celebrated example is Guanyin, the Chinese form of
Avalokiteśvara. In Chinese language, "having the heart of a Bodhisattva" is the highest praise you can give for a merciful person. I'm not sure to what extent such knowledge is presented in Western popular account of Buddhism.