THey have to publicly disclose if they own more than 5%. That's the level that's considered to be in the public interest. It's large enough to be able to influence the stock price.

However I do see why they permit companies NOT to report if they don't reach that threshold. If every fund or company had to disclose their exact holdings every day, a big company could hire enough people to enter all that data and then have a trading advantage because they'd know everyone's exact holdings. Over time you could see if a company was underwater on a position, and try to bankrupt them by taking one of their holdings down hugely. We have seen such an attempt made on IPIX. Huge companies with large staffs would have a gigantic advantage over smaller companies. Plus, someone's exact holding could be considered a "trade secret."

And, imagine the nightmare if you had to report that info every day.