This guy has done an admirable job of appropriating the visual style of Scott Hansen, of iso50.net / Tycho, who has been making design in a similar vein for many years.
Here are two designs I was immediately reminded of; both because of the style and, for these two in particular, the subject matter.
Even further back in the aesthetic pedigree of this work you'll note similarities to the work of Shepard Fairey, who created the OBEY giant phenomenon and, more recently, the iconic, 3-color HOPE poster for Obama's presidential campaign.
I don't think you can say someone is "appropriating" the style of a modern artist by virtue of using the general design aesthetic of the late 1970s. I followed your links and I don't see any particular borrowing beyond the general idea, which can hardly be said to belong to anyone in any sense.
The 1970s belong to us all†. For better or for worse.
(†: Certain aspects of the 1970s may not be available until 2065 or later, assuming no further modifications of the copyright act. Consult your representative for more details.)
Here are two designs I was immediately reminded of; both because of the style and, for these two in particular, the subject matter.
http://www.merchline.com/iso50/productdisplay.3969.p.htm / http://www.merchline.com/iso50/categorydisplay.2586.c.htm
I enjoy Scott's work and have purchased a few of his posters. This one is my favorites:
http://blog.iso50.com/wp-content/uploads/2008/12/iso50-north...
Even further back in the aesthetic pedigree of this work you'll note similarities to the work of Shepard Fairey, who created the OBEY giant phenomenon and, more recently, the iconic, 3-color HOPE poster for Obama's presidential campaign.
http://en.wikipedia.org/wiki/Shepard_Fairey