Why not ?
Why would I obstruct development efforts that I believe would
harm Bitcoin in the long run and invest resources for another fork to win if I wanted to Short?
Which brings us back a few posts... to me... the said contracts would make sense only if I believed that these large businesses pushing for the increase would end back on the 1 MB chain.
...still might be missing something though...
Let's assume the prices of bitcoin and gavincoin diverge because of the shorting and that you can sell one bitcoin for two gavincoins.
If you support the original bitcoin you'll mine on the original chain.
Now if you think gavincoin will prevail, it's
still more profitable to mine on the original chain, because whatever you mine, it will ultimately yield two gavincoins for every original bitcoin you mine.
And obviously, by doing so you're strengthening bitcoin at the expense of gavincoin. The value of a coin attracts mining, which in turns increases the value of this coin against the other.
What you are missing it that if the fork is actually made, blocks compatible with the 20 MB fork only will come after 75% of the nodes have announced themselves that they will be ready for the fork. So, either this happens and it's more profitable to mine on the 20 MB chain, or this doesn't happen and nobody will be mining on the 20 MB chain.