Because they are using document.write()
:
http://ads-by.madadsmedia.com/tags/22430/9194/async/160x600.js:
if (!window.ActiveXObject){
document.write("<div style=\"text-align: center; margin: 0px auto; width:160px; height:600px; position:relative;\">");
// etc.
If document.write()
isn't run in-line on and actively "open" document, it'll clobber what's there. So, running their script post-load overwrites your content with theirs.