They do not want to make search engines follow those link and share the link juice of that particular page.
On most blogging platforms the default setting is "nofollow" although this can be changed. However when you consider that a link is a vote for the receiving site it stands to reason that you do not wish to be handing out links too easily since the more links passed out the more it is actually diluting the value of a link.
Having one of ten links from a site is a lot more valuable than having one from a thousand links.
As a rule of thumb if you had a website or blog you would want more incoming links than outgoing ones but as a lot of people have found out is that a link with little or no value will actually hurt your site rankings.
A link from a good source is beneficial for SEO but by the same token a bad link will actually hurt your rankings. Hope this helps. Jim
Many blogs are implementing this to help reduce the sheer amount of spam currently being sent to the website. Many programs like scrapebox attempt to harvest these websites and leave low quality links, which can be annoying for a webmaster.
While many feel do-follow is much more powerful than no-follow, it becomes a prime target. Implementing no-follow makes the links less attractive to potential spammers.
Because they don't want to give their website's link juice therefore they give "nofollow" links.
I have been wondering why do they need to set it as NoFollow
