I'm with Mirikon here. To give another example (from 4th edition, but that hasn't changed) the Zoé evening gown (availability 10). Do you have to go through the black market to get one. Absolutely not. But say you have found the Zoé store in Seattle and you're inside. Zoé makes dresses to fit as they should (which is one of the reasons they are so expensive). So the first time you go there, they take measurements. Then they tell you to come back in three weeks to check on the dress (it will be ready in one, but to keep it exclusive and to make sure they can deliver even if there's a problem somewhere, they add two more). Then you come back and you try it on, after which they need two more hours to make those final adjustments. That's why it has such a high availability, not because you need to go through black market channels.
During those weeks they will also have tried to figure out if you can actually pay for the dress.
Another option is because the manufacturer wants to keep the product exclusive and/or drive up the price. Commlinks will probably be the best example of this. Think of a modern day iPhone. You can get one at a lot of places, but you often have to wait quite some time before one is available, especially at the start of a new product, because of slow production, shipping problems and the will of Apple to do it like this.
Third reason why legal stuff can have an availability is because there simply isn't much of the product, no matter what the manufacturer or service provider does. As example take a cruise to Antartica. The cruise season there is only a couple of months long and there are only so many boats allowed. Therefore there's a waiting list and high prices, which in shadowrun is translated into an availability rating. In this last case you might get jumped on the list if you know the right people and give them a little something extra, giving it a 'black market' like feeling, even though the product is completely legal.