But do they really, in all cases?

Due to the market share of AWS for many potential customers it will come down to a decision between

a) click here in the aws marketplace and have my service added to the invoice my company gets every month anyway

b) start the painful process of onboarding a new vendor, get a quote, have procurement haggle with them for months etc etc etc. (overstating it a bit, I know)

And that is where to me it becomes a question of using/abusing your position in the market to a certain extent..