I directly support the "opt-in" methodolgy. But here (for those asking) are the arguments against:
1. The current "web" economy heavily relies on it. If something like "opt-in" legislation is put forward, then that will have a negative impact on the web economy and "cost jobs" (while in reality, it does not have to) in this "tough economic landscape". No politician in the US will openly vote to reduce any jobs but "government jobs", and will certainly not vote against the interests of the big tech companies.
2. Opt-out is much more palatable because the companies can (claim "an existing relationship" - aka Zuckerberg), claim there was a past relationship and it is an oversight, or simply make them hide the data better. Opt-in presumably causes them to wipe the slate and start over (like that would really happen).
3. It creates the need for more FTC inspection and enforcement jobs (presumably growing the budget).
4. The legislation is likely to also cause an outcry and dry-up of lots of free (think Facebook) functionaltiy and entertainment currently available on the "web". I am fine with that, but the sheer number of clueless/thoughtless FB sheeple can easily be stampeded to write their elected officials in Washington to express their outrage over gaining their privacy in exchange for giving up Farmville or microblogging their family and friends constantly about any little thing that happens to them. You can't very well expect people to value something that they don't think about all that often over something that is the sole source of happiness and satisfaction in their otherwise insignificant and pointless lives.
I would also state that any legislation passed will likely be "token", "unfunded", and full of loopholes. Companies will also begin to trick citizens into accepting/opting in to tracking agreements in a wide variety of ways, including trojans and zombie mass-acceptors if need be.