"I am not going to let you sell your widget through Tamazon" is unfortunate and potentially crippling for the supplier, but as you've pointed out that's business.
"I am not going to let you sell your widget through Tamazon... in the event you make disparaging remarks about Tamazon in any public forum or divulge any information about our agreements" is very different.
In the later case, the last mile is threatening to retaliate in the event the supplier does anything they don't like.
Which, as a supplier, you rightly wonder "What can I do to avoid getting on Tamazon's naughty list?" To which Tamazon replies with a wink and a smile, "Just be careful what you say and to who, and I'm sure it won't come to that."
Absent legal protections and methods of redress, this dynamic is likely to exist in any market where the aggregator captures a sizable customer share. The size disparity is simply too great.