It doesn't taint the parent frame, but it makes the parent frame secure indirectly. The reason is if you parent a secure frame to something, then all parents up the chain become secure indirectly. That is, you aren't allowed to move/modify/change/show/hide/etc the insecure parents to move/modify/change/show/hide/etc the secure frame while in combat.
This is also why if Nameplates are on, Flash-n-shake doesn't work in Bigwigs/omen/dbm because nameplates are secure and they are parented to WorldFrame.
Is there a way around this secure indirectly? Like getting the frame's position relative to UIParent and placing the secure frame on top of it but still parented to UIParent?
SetPoint does not set a frame's parent. SetPoint merely determines location, nothing else. For the purposes of secure frames, both setpoint/setparent chains are all secure so that you can't go around the "security". If this isn't true, it would be true within a very quick patch.
A frame's anchor points determine's its location and possibly the width/height as well if there is more than 1 anchor point.
A frame's parent determines its visibility, scale, alpha, and relative framelevel changes.