I agree in both cases it is rude not to respond. But in one case the potential employer is the one who reached out first. For example, when I got out of college I brushed off the fact that an employer didn't respond to my resume at all. That didn't make me angry. What did make me confused was someone who specifically reached out to me to try to hire me through email and a phone call and then fell off the face of the earth when I followed up via email with a resume and more details. Sure that person might have been busy and had more important hires to make than a post-grad, but it was still rude and I probably won't forget it.