Rather than assuming it's either the sites or the developers policy - I would say if the developer is fine with reinstalling the software after the giveaway they would probably say so (if they don't actually announce this - it can't hurt to just ask them. At the worst they will say "no").
You might be suprised that a developer might even consider a significant price reduction - the choice between getting some money or nothing at all can sometimes mean getting a pretty cool discount. In my experience I have even been pleasantly suprised with a price reduction to "free". I might return the favor with testing new releases or making suggestions - I guess then it's not really free, I purchase the software in exchange for my time, experience and future assistance to the developer. But I don't mind.
I'm sorry if people disagree with me - but as I am a software developer myself, I can see both sides and think that if a developer really only wanted to restrict installs to that 24 hour period then basically saying "stuff you, I've got it now" and going against the spirit of the giveaway really isn't fair.
Still regardless of what people say or want, some people will do what they like without any thought for whether it is "the right thing" or not. Most people probably don't give it much thought.
The whole concept of this site really does work a lot better (and is more fun) when you honestly approach it as a "give and take" instead of "just take".
Instead of just complaining on the comments page, people could try writing an email to the developer and just saying "thank you" or telling them what you like (or not) about their program. It doesn't cost you a cent. Trust me, you will feel good about it - and it will mean a lot to the developer.
At the end of the day it's up to the character of the individual. I'm sure nobody is lying awake at night worrying about a pack of lawyers knocking on their front door - because they reinstalled a giveaway. Try not to focus so much on "How can I get away without paying anything. What's in this for me?"
Remember it is still it is worth trying the alternative of genuinely giving something back each day (GiveBackOfTheDay?). You might be amazed at the results.