That which is seen is the purchase of $0.50 worth of bitcoins for assets that only cost $0.25.
That which is not seen is the non-sale of those same assets for $0.50 worth of dollars (as other have pointed out). When you add that opportunity cost, the 50% discount vanishes.
That being said, we should bear in mind that business people will always prefer to pay bills in kind rather than in cash. For example: when Microsoft got fined billions and billions of dollars, they were very pleased to be able to pay part of that fine in the form of Windows installation CDs, which they were able to value at full retail.
The trick of course is that it doesn't have to be the same customer purchasing in dollars and in bitcoins. If the merchant offers to sell in both then he can increase his market share because the person who wanted to buy in bitcoins can be satisfied as well.
+1