1) they're earning money from this overall, otherwise they wouldn't be releasing more bundles
That doesn't mean it's okay to cost them money.
2) you might not have heard of these games/developers before, and will likely be enticed to go out and purchase other games by the same devs. Companies spend a lot of money on advertising. This is another form of advertising.
To an extent this is true, but not entirely. Humble bundles are where games go to die- it's their last effort to get money out of the releases, to get people that haven't bought them at normal price to buy them now. Titles don't sell almost at all after humble bundle releases. Some devs have even outright said that it's their last ditch effort to make a game profitable.
When someone gives out free samples at the supermarket, is it amoral to accept, because it's costing stop & shop (or whatever) money to give you the container and to pay the worker to stand there giving people stuff? No.
This is a bad example because 'free samples' at supermarkets and stuff are just that: free. They're not 'pay what you want'-you have no choice in the matter unless you try to tip the person. Speaking of which, that's a better example. The humble bundle is more like tipping.
If you give someone a tip of 1 cent, it's an insult. Paying 1 cent for a humble bundle is worse than that because you're actually bringing detriment to the people doing it. It's like saying "Thanks for the food and great service, enjoy the pile of feces I left on your table".
You're absolute scum if you do that. No amount of trying to rationalize it makes it okay.
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