Your suggestion that large companies ought to be "bound by law" implies regulations by the government. Whether or not that is your intention is still up in the air. However, if the government owns the regulation then they get to set the standards of what a big company is on a per criteria basis.

If the government is being attacked by a specific 'group', the government will then have the power to impose specific, targeted regulations on that group's web presence. Without the legislation, this is a matter of freedom of speech and is quickly resolved by the first amendment. It would be incredibly difficult to fight that off.

However, with legislation in place that affords the government the power to impose such regulations, they can quickly and most importantly (read terrifyingly) silence people on the web.