The incentive for retailers is to provide as little post-sales support as they can get away with, margin has nothing to do with it. Also, online stores don't tend to "advise" customers.
Hence RPM. If everyone is selling my product for the same price and making a healthy margin doing so, they have to compete on other factors, customer service being the most obvious.
Online stores often indirectly benefit from brick & mortar retailers - customers browse in store, then buy online. Again, hence RPM.
Online stores often indirectly benefit from brick & mortar retailers - customers browse in store, then buy online. Again, hence RPM.