It is the accepted way of indicating disinterest nowadays.
I've been guilty of this and also had it done to me.
It is annoying when people just disappear.

I have this also, I guess they rather do this than face the person, to me it's the coward way out.

I hate it when they do that. Just say sorry and I am not interested. Such an immature way to handle it.

"Is it just a fear of someone getting upset with you so you avoid the situation altogether?"

Yep. And it's so much easier and more impersonal to ignore online.

This does not just happen online.

I have had it happen after dating someone for a couple of months.
All of a sudden you just don't hear from them anymore.
Like they just fell off a cliff. It's really odd.

It is kind of a cowardess thing some men do.

When I was active in the dating world, it was almost predictable that the guy would say "I'll call you." And often HE NEVER DID. A lot of people hate confrontation or unpleasantness. It is cowardly, but somehow they feel it is more kind and socially acceptable. But if there has been a relationship (not just a few casual dates) and the guy "ghosts", well that's just plain heartless and rude.

I had a brief thing with a guy....he was a gigantic pain in the arse, anyway...when I go out I often see him, he never talks to me, but I get seriously stared out the whole time. Its so annoying. Ive tried several times to talk to him and get him to just be normal, or have a conversation, it never works. He is possibly the hardest / weirdest dude I have ever met.

Control. The GHOSTER desires to keep upper hand and keep the other on a string,, Loss prevention is another way to describe.

Thats so true Ceds.

Here's a better question to ponder: Why does it bother you?

You'll never truly understand what motivates others, however understanding ourselves is within reach and much more beneficial to our well-being.

It would bother you as when you become involved with someone then they start acting weird / dishonest causes emotional distress...it's a normal healthy reaction

yeah, plus when they disappear (out of the blue), you can't help but wonder, did they get busy, got hurt, or simply lost interest. You have no idea (it just happened out of the blue)? I'm not talking about people you chatted with online (never met face to face), but people you've been with a out a few times, pretty chatty, things seemed to be going good (perhaps you were intimate), and then poof they are gone.

Plus when it happens, at first you think, you doubt yourself, thinking, did they just get busy, or did something go wrong. You have no idea, if you'll ever hear from them again. Because they didn't speak up! Do you wait, or let it go...

It's such a bullshit thing to do and it is cowardly. Sure, the guy may not be on the same page, but I guarantee that it didn't take him that long to realize that. Besides being rude, the other person wasted their time that could have been spent elsewhere.

I find allowing my mind to chase for answers to questions I don't have answers for an exercise in futility.

I also don't understand how putting one's self into a negative affective mental/emotional state can be considered healthy, though I grant you it's probably far too normal a behavior.

People do what they do for their own reasons. If I understand, things are just as they are. If I don't understand, things are just as they are.