Sure, 'investment'
I'm a western expat in a developing nation, which probably doesn't help, but if I could see some, any kind of, results from proposals I make with free credits, I might accept the argument.
As it is, they are rarely even read, so it doesn't matter how human they are.
I get no actual work out of it, and I'm quite disinclined to pay to make proposals, when I would also have to pay a percent of any actual work I might 'win'.
Remember that most of the proposals clients get are bot-submitted ones, by people who don't have the least experience or capability to deliver what they're promising, and it shows. The competition is very weak, is what I'm getting at.
You have to pay to submit proposals, that's true - but a single job you score there will make it worthwhile. It's an investment like any other, and it ends up being much cheaper than adwords or alternatives for finding work.