One problem is that many people don't have bitcoin even if it is the best way to pay. Many people are uncomfortable buying bitcoins.
Amazon micropayment service lets you pay with credit card transaction anywhere from $0.05 to $9.99 with a 5% + .05 cent transaction fee. Which isn't as good as bitcoin but it is still pretty good as far as credit card fees go.
https://payments.amazon.com/help/Amazon-Flexible-Payments-Se...
Many small micro-payments can really add up. This can be huge for content creators. I would really like to give tips to people who create educational science videos, open source software, open source hardware, write detailed tutorials for hardware and software projects, take the time to help me on forums, etc. Many content creators on the internet have helped me. I might not want to give a lot to each but I can give a small tip. Which can still make a difference to the creator if many people also do the same.
I think if they charged a fee for the tip it would be a more standard business model and people would not find it as exciting. It seems more direct to customer this way without the man in the middle dilemma for content creators. Also charging a fee in addition to the transaction fee would make it increasingly difficult to tip the creator a small amount. I think the fact they don't charge a fee on top of the transaction fee is out of necessity.