Having majored in Economics at university, this all comes down to demand elasticities and available/acceptable substitutes. The number of substitutes, particularly for Bachmann locomotives more so than their Scenecraft range is low. For example, if Hattons don't stock the Bachmann class 04, you can't just buy a Hornby one instead as they don't make one. Will the demand for a Bachmann item go down if Hattons don't stock it? Unlikely, for their locomotives range at least, considering the number of well renowned competitors such as Rails of Sheffield able to supply them to us, the consumer. In other words, Hattons has many competitors offering the same product, Bachmann don't.
Most of Hattons sales are made online where the number of alternative retailers is high and a consumer can, with the click of a button, buy a product not stocked at Hattons from another website. The number of customers who shop in store at Hattons and think for example 'oh, they don't stock the Bachmann class 150 so I'll buy a Hornby 153 instead' is quite low.
Therefore, if such a dispute does indeed exist, it would be much more in Hattons interest to resolve it amicably.