You can't place a cactus next to redstone dust, but you can do the opposite without breaking the cactus.

Steps to reproduce:

1. Place a cactus on sand.
2. Place a redstone dust next to it.
3. Power the dust and see the cactus breaks.

What I expected:

1. Either cactus breaks when you place a dust, or
2. Cactus and redstone dust can be placed next to each other. Dust does nothing to cactus, regardless of whether it's powered or not.

I think (2) is the correct behavior because redstone dust is not solid.