People who buy her T shirts and other wares are getting something out of it already. The product. She has a business and she sells things. People don’t just buy her stuff to support a cause, they buy it because they like it and enjoy it - wearing it, having it, whatever.
If I go into a shop and buy something which is -essentially- exploiting a political interest of mine, (eg- a feminist card from Waterstones) just because it makes me smile, I don’t get all tied in knots, worrying about the person who might be making money out of me, worrying that they aren’t donating a percentage to a feminist cause or worrying however they’ll choose to spend the profit.
Some people donate money to her channel. And although there are many people who want it to go to a particular action, others will just want to support her for taking the time and effort to set up her channel, all the tech and paraphernalia that goes with it, showing up, and sharing her thoughts and activities. Many people will watch her and donate to her because they feel grateful that she is so fearless and articulate, able to say what they think, out loud. It feels like a service. The money they donate reflects the relief or joy she makes them feel when they tune in alongside all the others who feel the same. There are a lot of people who have made a lot of money through YouTube, twitch and so on, for providing a similar service and all they are doing is chatting to the camera while playing Minecraft - there isn’t any big cause- whereas- she does get important stuff done.
Me personally, I am grateful to her for taking the time away from her family and taking big personal risks, to herself and them, to get the message out there. I am very happy for her to get financially rewarded for that. I hope the money with help to safeguard her against burnout.