They are not buying end products. Germany supplies plants (!) for conventional military production. And Rheinmetal financed creation of testing grounds at Mulino.
http://jbpress.ismedia.jp/articles/-/44305. Despite official sanctions Germany and the US (!) continue to supply Russia. Just like the US sold to both Russia and Germany before WW2.
Military industries need to sell stuff. France, Belgium, the UK, and others do the same.