The reason for the 100 year mark is that the contract must have an end date. As a result, when the contract ends, that is to say that when the current owner (you) transfers title to the next person (whether it's family or not), the land lease owners can change the cost per year of the loan. The 100 year is also chosed because homeowners want to feel like homeowners and not condo owners. Additionally, they don't want their land lease fees to go up at some future time. Land owners get income, home buyers get security of prices. Mostly a win for the land owner, but the land leaser is motivated to keep the community a worthwhile investment.
Additionally, there may be some homeowner association restrictions similar to CC & Rs. In order for the land leaser to maintain the value of the community, it is in their best interest to keep things in check. For those of us that don't put obnoxious satellite tv dishes on the front of our house as an eyesore visible to the public, it is not a big deal. For those of us that might want to say change the color of roof, there may be a restriction. The same goes for putting solar panels on your roof - though there is clear law that says that no association can restrict installation of solar panels on the basis of community rules. Caveat emptor.