I think MANY experienced doulas, myself included, do charge for an interview. Just like some midwives do, just like basically all doctors do, lawyers, etc... The difference is that we generally waive the fee if we are hired - let's see if your pediatrician will do that
Here are my reasons for charging a fee:
1) I have to pay for childcare to go see you. That of course is built in to the cost of my clients, but since you may not become a client, that is money I have to pay.
2) I have to pay for gas to go see you. Have you seen gas prices lately?
3) I bring a LOT of handouts. They cost money to make.
Also, the other thing is that I don't want to waste my time with someone who might not be really interested in hiring me. I know it sounds kind of crass, but there are people who will bring in 10 people or whatever for an interview. I really try and discourage that. Mainly because I just don't have time to just do interviews all day.
Here is what I do do:
I will talk to you via e-mail/phone practically an unlimited amount for free before being hired. I say "practically" because in reality there could be someone who may try and take advantage of that, but I have never personally had that happen.
E-mail me - and your 10 other doulas - all you want. Ask me questions about practically anything. Call me and pick my brain. Call me a few times. See how we connect. I will do all that for free.
If you want to meet with me in person, it means that you already think there is a pretty good chance that you want to hire me (and I have a 98% hire rate among people I interview with). Honestly, it just weeds out people that aren't really serious about hiring me - and also helps them beforehand think about what they want in a doula.
All of that being said - those are my reasons, and I certainly can't speak for the doula you want to interview. I also only charge $50, not $100. And I have been known to waive the fee under certain circumstances.
I do hope that helps make it clearer.