That is probably how it seems to someone wanting Ceremonies tickets to an Olympics that's outside of their country.
However, for the residents of the host country that simply isn't true. London held back 75% and Rio are holding back 70% of their total tickets for the public (residents). With regard to the highest demand events like the Ceremonies and the Men's 100m Final session, then yes, the % of seats available for the public are reduced to something like 40-50%.
And of course, any London tickets, made available through the official LOCOG site were also availabe across the whole of the EU.
I know someone who managed to buy 20 Opening Ceremony tickets in one go via the official site - LOCOG deliberately held back 1 million tickets from the original ballot. And a large tranche of Opening Ceremony tickets became available through May/June 2012.
If I was a Brazilian resident, and I was determined enough to get there, and knowing how the process worked for London, then I'd VERY confident of snagging an Opening Ceremony ticket for Rio.
From the perspective of a US-resident wanting to attend Rio, that may be true, unless you have contacts in Brazil, Europe or Asia who could help you.
It's a shame that Cosport is your only option. No wonder the secondary market is so lucrative in the US.