I think offsets are an excellent way to keep some cause-promoters honest. For instance, if people who care about animal welfare are tempted to exaggerate the effectiveness or evidence base for animal charities, they might be deterred by the thought that people will make the obvious inference about offsets, and conclude that it's not worth it to give up animal products because it's worth more to them than the price of the offset.
It seems equally plausible that otherwise honest cause-promoters would be incentivized to be dishonest and downplay their cause effectiveness. In general, I don't think that assuming that everyone is a rational economic actor and speculating on their incentives to lie is very productive.
This works even if no one actually buys the offset - you can use this kind of number to help establish a preference ordering among very different uses of resources, like Katja Grace does here.
It's also worth just running the thought experiment as a check on your numbers - would you actually be happy if people gave $10 to CiWF instead of giving up chicken for a year? At what scale does this change?
Sure, that's fine, just not offsetting.