Why is my Arduino frying my lasers?

LASERS work perfectly fine from, and actually require, constant current drivers. You don't need feedback from the photodiode the alter current if you have adequate heat sinking and/or run fairly sort duration. If you are running long duration, like a DVD player or LASER projector, then you may need the use the feedback. But you might still get away with adequate cooling. I build multi-Watt LD pointers and only use heatsinking.