Personally, I do think 30 days is a long time for a buyer to be allowed to keep an item -  and it is rather inconsiderate behaviour to leave it until the very last minute to start a return, unless it is due to genuinely unavoidable circumstances.

And how exactly is it that you know that this is not "genuinely unavoidable circumstances"?
Your making assumptions!

It's not really very unfair at all.  It's a reasonable period to review the product and if it's a gift for instance, you have to wait until it's given etc, before it's even opened and checked properly.  And that's just one circumstance that I can think of offhand.
But then if it where a business, they are liable for a lot longer than 30 days, depending on the actual product.  ie. at least 6 months!
So if you think about that, then see your 30 days, is it really that bad?

As to a customer leaving it until the last minute, it's hardly unusual.  People forget to do things, then they are racing around at the last minute to get it sorted!  Whilst it can be a right pain for sellers, it is what it is.