Why was this problematic? (A.C.T. "libraria non grata"?)

One of my pages (hosted app) wouldn't display at browse-time, and I got an err msg about this line of markup in it:

<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="cc1" %>

It was the only page that had that line, so I stripped it out, and it ran fine. Two questions:

1) How did it get inserted there in the first place?
2) Why did it cause a problem - is the AjaxControlToolkit persona non grata by hosters (discountaspnet in this case)?
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