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The fact is that this is not true. Many vocaloid songs, not all, are taken as "royalty free" on Nicovideo. This means, it is often regarded that you do not have to pay copyright fees for them unless you use them for commercial purposes. True, there are many secondary songs and videos on Nicovideo. However, all the literary works are under Berne Convention, if the country where you are is a member of it.
The stance on You Tube reproducing more varies from creator to creator. Many creators are tolerant to copy their works onto the site as long as their names are credited properly, but you have to remember that some are not and that some request users not to copy their works without permission. In addition, many creators are negative to the redistribution of videos, mp3s, etc. However, since the copyright law of Japan has strongly protected copyrighted works even without being asserted or declared, writing terms of use or requests is not in common on Nicovideo and most of the Japanese creators have not thought that their works would be used out of Nicovideo. Furthermore, note a few Vocaloid songs are registered on JASRAC (such as doriko and 19's Sound Factory). The operator of Nicovideo (Niwango Inc.) and Youtube (YouTube, LLC) pay the copyright fee for the registered songs collectively and this is the reason you can listen to the copyrighted songs sung by vocaloids on Nicovideo or Youtube. Of course, you should not upload the commercial products, such as songs distributed only on the medium form of CD or i-Tune, onto Ustream and other video sites without authorization. If you are not sure of the creator's request and the copyright issue, especially you do not understand the credits and reminders written in other languages, it is prudent to refrain from reprinting the song.