well i can't find it on the forum ATM, but i have seen this said here! although a fully new 9V battery, not loaded can float up to even 12V , so when you hook that to your diode, it gets a spike higher than it is meant for. yes a resistor _may_ work BUT adding atleast a capacitor would work even better, but using this simple method, the diode will not last as long as it would with a proper driver circuit, and cooling. as i said above, people truly into lasers, want their diode to last as long as it can, whereas people who have never touched a laser, or are knowledgeable about them, will just use a resistor, since they don't care about the life time or performance, they just want to get it working.