Because what you are experiencing can be a side effect of Lasik surgery. I am very impressed by your responsible attitude. Prior to the cross linking procedure have you been wearing special lenses for your Keratoconus? Any type of smoking reduces perfusion and the cornea needs maximum oxygenation while healing. So play it by ear on your post surgical visits. If everything is healing quickly then you will be able to have some sooner but I think your one month estimate is a good, conservative approach.
If you have any complications crop up the last thing you want is to have them try to fob it off on smoking marijuana. Plus you have the rest of your life ahead of you. So a 4 to 6 week break couldn't hurt. If you are an occasional smoker than stopping 3 days prior is probably ok. But the best thing would be to run this by your opthamologist who knows you.