The purpose of the leverage points is to change the system at a fundamental level. Changing taxes doesn't (until you hit extremes) cause that. The system continues to behave in the same way but with increased or reduced constraints. Consider birth control policies. Eliminating birth control (making it illegal, so not really eliminating it) can cause an initial increase in children born until a new stable point is reached. Once you make birth control legal again, unless there has been a culture change in the interim, the birth rate will drop to the prior level.
Taxes and subsidies are the same. They don't change the system at a fundamental level (or don't do so easily), they only alter its external behavior. But once they're restored to their prior levels the system will probably return to its same behavior.
As a tax example: Perhaps we want to encourage people to save money for retirement, so we give them a tax break on their IRA contributions. This seems to help, people are saving more for retirement (as measured by IRA contributions increasing). We remove the tax break, do they continue to contribute (it's now a part of the culture) or do they stop contributing (they only did it for the tax benefit)?
If they continue, then you managed to create a substantive culture change. But most likely you'll get a drop of contributions.
[0] http://donellameadows.org/archives/leverage-points-places-to...