It sounds like you are trying to move DRM'd books you bought from Amazon to another Kindle, which is indeed not possible – that is the purpose of DRM. You'd need to strip the DRM for that to work.
But as other commenters noted, if you sideload ebooks which do not already have DRM on them, the Kindle will certainly not add any sort of DRM to the files. This is true both if you sideload via USB or even if you use the "email to Kindle" feature.
But as other commenters noted, if you sideload ebooks which do not already have DRM on them, the Kindle will certainly not add any sort of DRM to the files. This is true both if you sideload via USB or even if you use the "email to Kindle" feature.