They wouldnt be able to stop everyone from installing it obviously, but the damage that could be done to bitcoin if just running the bitcoin client is deemed illegal is enormous.
Im not looking forward being put at risk of breaking laws from just about every regulator in the world and Id rather not risk finding out what exactly it is they could do.
You think the SEC has control over what software people install on their computers?
Bitcoin can already be used to transfer money to terrorists and drug dealers. If you think those things are less of a problem for the government then unregulated securities you're definitely smoking crack.
Unregulated securities directly threaten the very lifeblood of governments -- money. If you think that drugs or terrorists are a greater concern to governments, you must be new here
*That said, i doubt colored coins will be the undoing of bitcoin.