I've been using Coinbase for a few months now and noticed this as well. I would imagine it's simply their way of making money (they are a business after all).

I really don't have a problem with it (they can quote me whatever they want, doesn't mean I have to buy it).

The only thing I really care about is that they're honest and clear about what they're charging (so far haven't had an issue with this).

Now, if they start pulling this "High Risk" BS I've been reading about, that will be an entirely different story. Luckily I haven't experienced this yet either (although my purchases so far have all moved against me during the clearing period).