I'd agree they should not have bought there, but unless the community association dictat says specifically "Homeowners must all maintain x percent of their property in lawn of a,b, and c types of grasses mown to z height, and Texas bluebonnets must be confined to flowerbeds," homeowners have no way of knowing that the community association Nazis are going to have an issue with a particular type of vegetation.
We had some neighbors who put in Leyland cypresses to maintain their privacy. The community ass'n and its attorneys really had a field day with this. It was unattractive and not in keeping with community standards, they said. They racked up some big ol' legal bills, too. Unfortunately for them they'd picked on a doctor who had the money to pay for his own lawyer and he fought back. His lawyer pointed out that there was no place in the community documents that stated Leyland cypresses were not permitted and the community association could just go pound sand. It was nice.
The simple fact is that when you sign a CONTRACT [aka the HOA agreement], you are agreeing to abide by the laws now and in the future as long as you live there.
The rules are changing contants in most HOAs. If you can’t handle it, leave.