There are vast differences in government "support." In terms of financing aircraft sales through a country's development bank - and all companies do this - there is no real problem. When you talk about tax breaks and incentives to locate or expand a factory, again, all industries receive the same consideration.
The real problem is in governments - taxpayers - funding development programs. This is what Boeing and Airbus are discussing now, and this is what Bombardier plans to do to develop its CSeries. Under the 1992 bilateral agreement between countries representing B and A, up to one-third of development funding could come from governments. That covered only countries signing the agreement and it has been declared no longer in force by Boeing, who wants a new agreement and is threatening WTO action. The US could also include Canada in this claim due to government support of development funding for BBD. Brazil would probably join in.
Here is the issue:
Market forces support market requirements. If a product is desired by the marketplace, the market - through various ways - will fund development. Investors, banks, shareholders - all would see the potential value and fund the program. When governments get involved in development, it becomes a jobs program and there is no real indication of true market need. A company will claim they will repay the "loan" when the product becomes successful, but without a market-driven focus, it may or may not be a success. The taxpayer is stuck with the bill. Also, the company receiving the aid spends less of its own money on NRE and they are able to undercut the price of a competitor product because of this. This is clearly a violation of WTO guidelines.
Companies that make widgets don't get help to design and develop a new, improved widget - but they do get tax incentives to build the widget factory and to export those widgets. It should be the same with aircraft.
This is going to be a major issue between the US and Europe and I will bet Brazil and Canada get drawn into the fight.