Why is my Arduino frying my lasers?

No, not really, as stated above. nobody who builds hobby, multi-Watt (optical power), bothers to use feedback current limiting. we just rely on accurate constant current drivers and massive heat sinking, or, active heat remediation. even the hacks at Wicked Lasers didn't bother with feedback limiting.

Replacement cost of multi-Watt LASER Diodes IS SCARY to me, at $60-100 USD ea.

But, for the cheap little 5mW and below pointers, no big deal. rip one open and the only current limiting you are likely to find is a resistor. applied voltage isn't that big a deal either, as long as you are above the LASing threshold Vf (that is what the LD drops, not an absolute max.).