But are transactions from Bitstamp really free of charge? And so why? And why Kraken an Bittrex have fixed fees, it would be better if the fee depends on the amount.

If I want to move for example my bitcoins of 0.03639527 BTC (100$) from the exchange to my wallet in will pay a fixed fee.
Kraken - Bitcoin (XBT) - ฿0.001 ($2.75) https://support.kraken.com/hc/en-us/articles/201893608-What-are-the-withdrawal-fees-
Bittrex - Bitcoin (XBT) - ฿0.001 ($2.75)
BitStamp - Bitcoin (XBT) -Free of charge!!! - https://www.bitstamp.net/fee_schedule/
How is possible that BitStamp transactions are free if any coin has a network transfer fee? And why are Kraken and Bittrex so expensive if you make small transactions the fee charge is very high. I'm talking here abount bitcoin but for others altcoins the fee is high too. Let's say I deposit 100$ on Kraken. And if I buy 10 different coins for 10$ each and I want to transfer them to a wallet (or more wallets for different coins) I will have to pay about 10$ of fees. Is my thinking right? So the best option is not to transfer to wallets if you don't have a big enough amount because the fees are fixed or use only Bitstamp. Or buy just IOTA that has no fees.