Processors have upstream costs to Visa/Mastercard/etc that are charged in similar ways. The idea behind them using a percentage is that it relates to the risk. The more you spend, the more could be fraud and might need to be written off. The $0.30 is effectively a lower bound to stop micropayments. Otherwise people could do a $0.10 transaction and only pay $0.003. This could be to avoid system load.
While it might seem expensive, until a few years ago, taking card payments required getting a merchant account at a bank with high monthly fees which could take months. Now you can pay similar rates but without the misery of dealing with the banks.
Are you from the US? If so, that stuff about merchant accounts isn't true. While they're backed by a bank (acquiring bank), you don't have to go to a consumer bank to get one. You can go through any company called an ISO. In my opinion banks are the worst place to get one. We do merchant accounts with a monthly fee of $5/mo and instant setup (you can start processing in less than 2 minutes from when you complete the form). All backed by our friendly customer service team in Ohio.
Now if you're in the UK...things are very different :).
PayPal has a micropayments account option that it doesn't publicize very well. If you set up a new account with their special micropayments link (google it) they charge 5 cents + 5%. Which makes it cheaper for transactions under $11.
While it might seem expensive, until a few years ago, taking card payments required getting a merchant account at a bank with high monthly fees which could take months. Now you can pay similar rates but without the misery of dealing with the banks.