This is much more of a problem. You can't switch jobs every few months and expect people to continue hiring you.
Contractor can and usually does take on new projects all the time. So as a contractor when I hear 'Can you do X for us', the answer is usually yes, if it comes within our expertise area, but as an employee, especially freshly hired, you can't respond to every job interview request unless you really really intend to leave.
It really depends on the job you're leaving and the job you're going to. I often respond to interview requests, though I basically start the conversation with "I'm not really looking, but I don't mind talking to you." I'm not really an entry level employee, though.
