It appears there is such a restriction for equity securities:
- http://www.sec.gov/divisions/marketreg/subpenny612faq.htm
And exchanges such as NASDAQ had to request permission to do sub-penny:
- http://marketswiki.com/mwiki/Sub-penny_quoting
But that is for equity securities. Perhaps BitFloor and Camp BX are using a dose of caution, or just chose penny increment so as to not give bots / automated trading the advantage that sub-penny provides?
I watch the bots on Mt. Gox sit just above my bids or just below my asks and feel frustrated when theirs will trade and mine don't, thanks to decimalization precise to 5 or 8 digits. On the other hand, I've benefited from trading where I've been able to get in just ahead of the bid wall or ask wall thanks to decimalization where instead with penny increments my order might not get filled if I was among the most recent orders at that price and the wall doesn't get broken through.
So I guess I'ld prefer the exchange to not have any restriction as sub-penny works for me sometimes.