Market manipulation is the interruption of a fair free market through attempts to mislead or lie to people.
What BITMAIN is doing here is simply deciding which coin to accept for their products, which in itself is objectively not a market manipulation tactic nor any sort of problem at all. If anything, it's a good example of a company exercising their right to make business decisions like this.
BITMAIN know that there is high demand for their miners and they are using this demand for the gain of a currency that they support. That is perfectly legal and also ethically fine.
If you want to pay in BTC, you will have to just complain to BITMAIN or buy from a different company.
You seem to be quite defensive towards Bitmain's inevitable tactics.
I have no problem with them accepting Bitcoin cash exclusively to thrive their business but they are certainly misleading the potential buyers. If not, then they could have only mentioned their products' prices in BCH only (which they didn't).
If you still don't think they are taking advantage of their monopoly to gain BCH's traction, then you certainly don't understand how big whales control the market!
Can't wait for Bitcoin to change its POW algorithm to put an end to this dirty tactics...