It's definitely a balance thing. Back in early WOM hills did not have a movement penalty and raise land was only 5 or 10 mana. I routinely cast raise land to speed travel through forests.
There are more options later in the magic tree but not that cheap.
A grow forest spell would be a nice addition though, high in the earth magic levels.
It can't be too much of a balance thing, as it's already possible to deforest at will, it's just clunky and expensive to do. I'm not even overly concerned about the cost. You could make a dedicated "de-forest tile" spell, and split the difference for a cost of 75 rather than two spells for a hundred, although I think 50 is probably more reasonable. I dont see it as a casual early game expenditure. By the time you have enough mana to throw around on stuff like that, you have the ability to mess with the AI in far worse ways than forest exploits.
Now, I'm a believer that terraforming (and probably all Strategic spells for that matter) should be limited to two per turn, like some already are. I'm sure you can do cheesy stuff with it, but you shouldn't be able to do it in a single turn. Terraforming should be a gradual process, except in the case of something like Raise Mountain, where you're paying a premium for expediency. Obviously owned territory restrictions should apply, just like with Raise/lower land.
If I'm not going to have any control building my own roads, I'd at least like to be able to efficiently clean up when the AI makes a boneheaded decision on where to put them. I dont think being able to clear a path between your cities for a reasonable expenditure of cost and effort is unbalancing by the time you have the mana to actually do it. And think of all the pretty hedge rows and defensive forest walls you could build around your cities with a grow forest spell! I like to keep a tidy empire.