I have nothing against them selling the plugins. They can do whatever they want, and charge how much they want. The problem is when they have the monopoly of the market.
It still exists (at least for me), a huge gray zone on what I should expect to be included in Corona SDK and what not (that I should buy from 3rd party). Even the difference between the licenses (Pro x Enterprise) are not well defined. Today a feature that is not in the Pro version, is because it is in the pipeline to be or it will never be? Same for the plugins. If you remember, Facebook started as an API and now is a plugin. Same for Google Play IAP. What happens if Corona decides that it will now be a 3rd party plugin that you have to buy from these guys?
Imagine if you had to pay 50 bucks for each plugin that you use in your app. On a normal game I use: Game Center, Google Play Games Services, Facebook, Amazon IAP, Google Play IAP,… Do you get me?
In my view, Corona should improve its SDKs with new functionalities without increasing the costs to the developers, that already pay a subscription that is not cheap.