Consider that some of these mining firms have other revenue streams. You cite BitMain and it is an absolutely perfect example. Considering the amount of money and profit they make selling mining gear. Do you actually believe that they need to sell any % of the bitcoins they mine to cover expenses?
Selling mining gear is a welcome addition to mining yourself (unless it dilutes you too much via difficulty, an OCDM would likely frown on selling to the public), Bitmain's prices seem to track what the device might return before it becomes inefficient. It's almost like selling a bond to another bond investor.
Another example is BitFury which is getting fed by VC money at record rates and have also, in the past, had their fair share of surely spectacular revenues and profit selling their mining gears. Now we don't actually have to speculate about them since they have made it clear in interviews they are absolutely not considering selling any of the bitcoins they mine.
These together amount to nearly 25% of the network. Would you like to suggest they are the exception?
VC funded mining operations as the new 2012 guy with GPU's mining at a loss... I like it.
For everyone else, profit vs loss is a more tenuous balance, and as a whole, I think the industry sells more as a % vs the hobbyist days.