[laumars]

It doesn't. Sampling is technically illegal. However the bootleg market is often too small to go noticed and larger artists get authorisation before sampling (or at the very least - releasing).

You do often see some artists turn a blind eye to sampling though. Particularly dance artists because many of them know their entire genre exists of the back of sampling. So it would be counterproductive / hypocritical for them to chase after royalties

[decode]

It's not quite that clear-cut:

    Sample clearance is generally not required if:

    - You are just using the sampled music at home.

    - You are using the sample in live shows. This is because, 
      usually, you are not making copies and the owner of the venue
      pays the blanket license fees to performing rights organizations
      such as Broadcast Music Incorporated (BMI) or American Society of
      Composers, Authors, and Publishers (ASCAP).

    - You plan to distribute copies to the public but meet one of the
      following: (1) an average listener would not notice the similarities
      between your end product and the sample, or (2) your use of the
      sample falls under the "fair use" doctrine. For more information on
      these, see "Defending a Lack of Sample Clearance," below.

http://www.nolo.com/legal-encyclopedia/permission-sampled-mu...