The Supreme Court will soon consider whether resale price maintenance (RPM) should be per se illegal. Since we don’t understand price stickiness well, it is no surprise we also don’t understand RPM very well either. My seat of the pants guess is that > 50% of RPM represents a desire to collude and raise prices.
Market-oriented economists often exaggerate the "ancillary services" hypothesis developed by Lester Telser, namely that RPM keeps services flowing ("informative stereo salesman" is the paradigmatic example). Supposedly, without RPM everyone would get sales help at the expensive store but buy at the discount store; in the equilibrium all stores would be discount stores and poor consumers would wander through the world without sales help. In reality RPM has often been used for lots of the small, stupid items you see for sale in drugstores.
I also attach greater credence to the Ben Klein hypothesis that RPM represents a kind of "efficiency wage" to discipline retailers and force them — through threat of product cut-off — to present the item in a desirable fashion. (In Telser’s hypothesis the services flow automatically after RPM is instituted, through a desire to capture customers and extra profits, but why should the extra services supplied then be the ones subject to the free-rider problem, rather than some other side benefits?)
Even when cartellization is the motive, I do not worry that Colgate will manage to monopolize the market for toothpaste. There just aren’t many retail products which don’t face lots of competition from alternative manufacturers. I also don’t think that so many business decisions should become primarily legal decisions; our government has enough real crimes to look after. So RPM should be close to per se legal (certainly not per se illegal), with some possible exceptions for resource-based monopolies, not that I can think of any offhand in the retail context. Arguably government should not enforce RPM agreements, though product pulling is in any case the major means of implementation.