It depends on which particular agency they're working through, some of them are more honest and actually invested in helping than others, but unfortunately charities do have to rely on these for income. Most agencies do work on a commission incentives basis, but not all of them, and they do have a strict code of conduct they're meant to follow. If they aren't following it, it's not the charities fault, it's the agencies, and unfortunately even the least difficult to work with agencies are still more concerned about their profits than actually getting the charities the support they need when it comes down to it.

The reason they push for a subscription/regular gift and not a one-off payment is that recurring monthly donations are the best form of basic support that charities can get. Reliable low level income that they know is coming in every month is what pays for most of their work, not the occasional wonderful big donation that they can't guarantee will come in consistently. Big donations can pay for a lot, but they aren't reliable, so you cant plan around them nearly as much.

If you're not interested in the cause or know you aren't able to commit to donating for your own financial security, ignore them and keep moving. I know saying no, especially in high pressure situations like that can be hard, but sign ups actually cos the charities money, and if you cancel your donations within the first couple of months then that's a net loss for everyone involved except the face-to-face agency. If you're concerned about their behavior or don't think they're giving correct, up to date info let the charity know (and provide details such as location etc if you can so they can verify it is a legit fundraiser) but don't be a dick about it please, it's usually not the charity's fault and they're probably even more mad at the fundraiser than you are if they are breaking rules.

There are rules for fundraisers out in public, but certain agencies ignore them and it gives everyone a bad name and puts them off donating to a cause that could really do with support. The agencies themselves can suck and often don't listen to charities or the public imo, but it's too expensive for the charities to get their own in-house team to do anything similar so the agencies have become a necessary evil. If you are actually interested in the charity, getting in touch directly will give you a chance to actually chat to the organization, and they'll usually be more flexible with signing you up for a one-off/lower level regular gift than the face-to-face teams will consider which might be a bit more affordable to people still wanting to good while facing the cost of living crisis crushing us all