(note: this response is not directed at any single person - but at the multiple times this question has been asked).
Is it legal? Why don't you contact the developers and ask them. (Maybe the developers will just make their software freeware - and give up on the idea of trying to make a living from writing software).
The developers agree to a "time limited" offer allowing an install of their program during a single 24 hour period - coming into these forums and asking "how do I get around this" isn't going to get the answer you'd like.
I'm sure there are people trying to screw the deal over for anything they can get. As I've said before, if you start handing out $10 notes - there will always be people complaining they're not $20 notes - and they will work out a way to line up multiple times to make the deal sweeter for themselves.
Rather than going to whatever lengths possible to avoid having to face the possibility of paying for software that you actually like and use - if it really hurts that much to put your hand in your pocket and pay a few dollars to support the people who wrote the software for you - then the solution is NOT to work out ways to bypass how GOTD works, allowing you to continually reinstall old versions of programs - but to look for alternative Freeware and Open Source software. We love Open Source and Freeware software in these forums!!
And for the developers who might be considering putting their latest software creation, utility or blockbuster game on the site for everyone to try for free - might "scrap the idea" because of users who are only too happy to take a generous offer - turn it on its head and milk it for 200% of what it's worth - at the expense of the very people who make it possible in the first place.
You get a great deal from GOTD - but some people are just "glass half full" types. The Giveaway deal is what it is. If you can't live with how it works, don't ruin it for everyone else.