An important part of Wikipedia is that it is available under the General Free Documentation License. The practical upshot of that is that anyone can host a copy of Wikipedia if they link back to the original articles, with the appropriate copyright notices.

So, most of they sites are probably legal, although some websites have been known to use Wikipedia content while not following the rules.

Having copies of Wikipedia articles can cause problems in some cases though. If you are checking a more obscure article you come across in Wikipedia, it doesn't help when many of the websites you come across in a search are copies of the article you are trying to verify. It's generally avoidable though by inserting -wikipedia (for Google) to remove the copies.