The Inverse Square Law applies pretty much only to single point or isotropic energy sources. LASER energy is highly directional and moves in near parallel paths. There is some degree of divergence in LASERs. Researchers relied on some of that divergence in hitting the LASER target experiment left on the moon during the Apollo missions. You could imagine how much more difficult it would be to try to hit a small object with a very tiny beam at such distances and speeds.
So the Inverse Square Law:
If a point source is radiating energy in three dimensions and there is no energy lost to the medium, then the intensity decreases in proportion to distance from the object squared. This is due to physics and geometry. Physically, conservation of energy applies.
So light from a single point source travels outward in a sphere so as the distance from the source increases so does the surface area of the sphere. So in a sense the density of the photons would decrease as the surface area increases. Imagine a balloon painted with polka dots: as you inflate the balloon the number of dots remain constant but the space between the center of the dots increases also. This is not a perfect example - you would have to imagine that the size of the dots remain constant also.
A practical demonstration of this would be to take a conventional light source such as an ordinary incandescent light bulb. Cut a circular hole in a card opaque enough to block the light from the bulb. Keeping the distance constant between the light source and bulb observe the diameter of the "spot" of light passing through the hole on a card at various distances from the light source. The area of the "spot" should increase or decrease in accordance to the Inverse Square Law. Notice also that the "brightness" of the spot increases or decreases also. See the diagrams here.
The diameter of a laser beam for all intents and purposes in the distances involved for indoor plant growing will remain constant - thus the Inverse Square Law would not apply.