The main reason is because bitcoins are volatile. Most currencies are fairly stable, there won't be much fluctuation over the course of a week or month, for however long refunds are valid. This isn't true for bitcoins. Once you pay for something in bitcoins, you are trading those bitcoins for something of similar value at that time. I don't know what the bitcoin exchange rate is right now, so I'm just going to make up some numbers for an example. But say you buy a TV that has a value of $1000 USD. Let's say the current exchange rate is 10 bitcoins to 1 USD. Then you have to pay them 10,000 bitcoins for the TV. Say a month later, you decide you want a refund. So you return the TV. Well the TV still has a value of $1000 USD (ignoring deprecation) so you should get that same value back. If the bitcoin market has skyrocketed, to say 5 bitcoins to 1 usd, then you would get 5000bitcoins back, because those bitcoins have the same value. The merchant isn't acting as a bank to hold onto your coins for you, they are trading you something of value for something else, and if you want a refund, you'll get something of the same value back.

Considering that customers generally get to choose whether and when (within a certain window) to ask for a refund, refunding the same amount of BTC gives customers a license to steal.

If you visit a different country and pay for something in the local currency, and then later on you want a refund, you wouldn't expect the shop to refund you in your home currency?

Too late to edit, but I realize the problem is more likely to come up in the face of sudden and significant deflation, which isn't something we see in other currencies terribly often.