What put the scientific laws in place? Why does life exist? Why does it mutate, allowing for natural selection?
Noticing and documenting observations on how something works is not the same as figuring out why and the origins behind it.
Things mutate because different combinations of genes create different outcomes. If I smash apart two Lego models, 1 red and 1 blue, and make a new one using at least 50% of the pieces from each, then it's impossible for the outcome to be the same as either of the starting models. It will be a mix of red and blue. Now, I don't know what arrangement the red and blue pieces will be in, but they will be in a new arrangement, maybe good, maybe bad.
This is not an example of "allowing" the outcome to be different. It HAS to be different. So if you have sex with a woman from across the world, and the resulting child ends up with traits from you both, nothing "allowed" that child to be different from an original copy. By combining DNA from both parents it HAD to be different -no guiding hand was necessary to "allow it".
Say in the above example, there are 2 kids. 1 is is genetically lucky and grows up to be Arnold (the legos fell into the right places that day.) The other looks like a Mongoloid and ends up posting on NeoGAF. Natural selection has taken its course and done its job. Now stretch this over many many thousands of years -the better traits have a greater tendency to be passed down. You can play with the possibility that this system was set up by design, the thing is though, it really wasn't necessary at all for it to work.
The other aspect is that DNA takes damage, and there are also random hiccups as genes of different hosts are spliced together. This doesn't really need some big crazy explanation. Just see the random, chaotic way the world works. Every small bit of order is a valiant effort against the reality of total chaos. Spill a bottle of water on the ground and watch the randomness of the splatter landing in 200 places. The truth really is in something so deceptively simple as that. God's Hand was not necessary to guide the water to randomness and nor was it necessarily to create randomness in the gene pool either. Of course I can't
disprove that every time anyone has spilled a bottle of water, God didn't control it just because he likes to do it. But it sure as hell was not needed! You could say that gravity was necessary, but even if you make the case that God set up all the rules like gravity, it wasn't necessarily done just to make sure all the water splatter ended up in juuuust the right places He wanted.
The rules weren't necessarily made just to ensure mutation could happen either.
If you need an explanation for what allows mutation, you need an explanation for what allows wear and tear on my tires. Even if there is a God and he personally made all the matter than went into the aspalt and rubber, I don't think he was thinking about this or trying to make "wear and tear" possible. It doesn't require or demand an explanation in order to make sense.
Most mutations are not beneficial and will not make you a Ninja Turtle. They're bad. For that matter, doubtless many mutations have occurred throughout time that could have been beneficial to the human genome but were stamped out to extinction before doing so.