Iv been looking into the legality of codecs in the US and have found that many of the ones that I commonly use under Windows are illegal to use under Linux, without paying for them, due to copyright infringement. I'm just wondering if Blender suffers form the same type of problem in the US, and if it doesn't then how does it avoid this when Linux can't?

Generally speaking Blender uses open source, GPL'd or similarly licensed material that why they "can do it" as you put it. You can't use Window stuff under Linux because that material is licensed to or by Microsoft who basically 'wall in' their property. Licensing and codec support aren't necessarily mutually inclusive.
_________________
http://www.katsbits.com