I get a little frustrated every time I see people confidently asserting this. The assumption seems to be that because there was no market in the past this will be true forever...after all, if it were possible wouldn't somebody already be doing it? This same argument could have been used prior to the advent of the transcontinental railroad or the internet. In each case it was difficult to see more than a marginal business case before it happened and took off. The same is true with space. That being said, there are a lot of smart people whose job it is to look closely and try to see a pathway to a genuine market, who think it can happen. There are a lot of Newspace players I could cite, but since they are easy to dismiss as dreamers, lets look at Boeing instead.
Boeing is a conservative company and I think if they are going to do the CST-100 they are going to want a fairly fault tolerant business case to help mitigate the risk of fielding it. I think they are willing to give it a shot (in competition with 3 other players) only because there is a mixture of different market possibilities which provide redundancy if any single market does not work out for them.
-NASA ISS servicing
-Future non-ISS NASA missions (if Orion never flies)
-Sovereign Clients to Bigelow stations
-Tourism to Bigelow stations
-Private research to Bigelow stations
This last one is something which I think is often overlooked and which could be bigger than people think because I think many people ask "How many companies would have both the big $$$ and the research needs to rent a module and fly their own astronaut?". I think this question makes some fundamental assumptions that are probably wrong and consequently leads to the answer (not very many) which causes this type of demand to be sidelined in the discussion.
The more likely scenario is the rise of some companies that act as middleman human tended in space lab operators. These companies are the ones holding the leases with Bigelow and flying the astronauts, and then they turn around and provide a turnkey, low hassle, cost effective, user friendly way for companies and universities to get their research projects flown. Because the projects are paying for only what they need and not having to personally manage astronaut staffing & station leasing, the market is open to a much broader set of users than might otherwise be possible.
Because of the commercial nature of things, I am sure Bigelow and these middleman companies will be happy to keep CCDev craft flight rates and station facility sizing in line with the demand from the market so there won't be long waits in line for research projects to fly like you've seen with ISS and other options which have been available historically. Potientially this could cause what has historically been a fairly minor market to bloom into a much larger one.
Don't believe it'll work out? Have a look at the success of Nanoracks on ISS:
Beyond that, history does seem to give us a high degree of confidence that there will be at least a minor tourist market of a couple people per year based on flights of anousheh ansari, charles simonyi, dennis tito, eric anderson, greg olsen, guy laliberte, mark shuttleworth, etc. particularly since Bigelow would be cheaper than a Soyuz/ISS trip. Beyond that, there even seems to be a market for beyond earth tourism: http://www.thespacereview.com/article/1855/1
Even if tourism is a relatively modest business, it adds to the cumulative business case provided by the research market.
Then there are the soveriegn clients: http://www.space.com/9358-bigelow-aerospace-soars-private-space-station-deals.html
It is unclear at this point how large this market is, but it looks real enough to at least allow commercial operators to bootstrap the facilities online. Once these facilities exist, paid for by the early anchor tenants, they will be able to foment other uses and markets. Even if it is a relatively modest business, it adds to the business case provided by the research and tourism markets.
The point here is that even if none of these markets is initially strong enough to stand alone, in aggregate they may well be enough to build out private capabilities and start driving down costs enough to cause an iterative ramp up of strong markets. The transcontinental railroad did not initially go to huge cities, it enabled empty places to become towns then cities, and in so doing bootstrapped more of a need for the railroad which in turn strengthened its own business case. Initially running a railroad out to a largely empty area didn't have a completely obvious business case. It only became obvious once the feedback loop was truly running. The same is true of space.
Even NASA has acknowledged that there is real potential in the commercial market. Have a look at this: