Downloading datasheets and making them available on ones own web site is no different from downloading photos from someones Flickr, Photobucket or similar and then presenting them on your site.

The documents are not in the public domain. Public domain is a specific term where the author has explicitly released the information with a "do whatever you like" grant. The datasheets are only free as in free of charge. But the owner still owns them and is only allowing you to retrieve copies of the documents without paying for it. And the owner is allowed to change the licensing terms at a later time starting to collect money. But even if the owner doesn't, he must explicitly grant you the redistribution rights before you may legally distribute the datasheets on your own.

As Andy notes, the manufacturers may not be too busy hunting illegal distribution of the datasheets. But they have the legal right to do so, which means that it is dangerous to redistribute the documents without having a written agreement.