Come to think of it, how large must an organism be to serve as a viable host? Are cats too small? If so, I imagine they're not perceived as a threat either.
I agree about Dallas, but a great many of us simply didn't know what the alien did to him. It was left up to our imaginations. We hadn't read the novelization so his fate was left mysterious. For all we knew, the alien broke his neck and dragged him away for dinner.
Hence, the sight of all those colonists frozen in their twisted and terrified death poses was pretty shocking and scary to me when I was 11.