It's a scam in that it is actually a transfer medium with no reason to increase or decrease in value. The only way its value can go up is if people are hoarding it in such a large amount that it's hard to come by (like tulip bulbs or gold, which is also mainly a scam).
It is actually very easy to see why it increases and decreases in value and it is the same reason as gold, simple supply and demand. To put that in context there are only just over 16 million of them currently in existence and the bitcoin economy is booming as far as transactions go with somewhere around 150,000 of them exchanging hands every day.
So, supply and demand + utility. Exactly the same reasons that gold has fluctuating value (or value at all).
But the fact is that people view it as money backed by a government (which can increase and decrease value depending on financial situations in the country) or as a stock. But stocks increase in value due to increased productivity (well in theory at least, actually perceived productivity).
I don't follow you there, I am not sure that anyone thinks that bitcoin is backed by a government, that is kind of one of the strengths to bitcoin.
Bit coins as an investment is a scam.
Who do you think is running this scam? Is it any different than gold as an investment?