Tax evasion. Lots of contractors do this. Project will cost X if paid in cash, X+Y if by check or any other traceable means.
My thoughts as well
Probably not the smartest thing they ever done in this mess is to talk to the press about paying for the landscaping with cash.
They could be hearing from the tax men (local, state and federal)
Tax man is going to get his cut.
Not tax evasion by the customer, who wants a better deal. Same goes on at stores and restaurants. We always pay in cash, many clerks say they have to pay 3% or more for credit card fees so they discount if you pay in cash. If a contractor gives a discount, who are we to blame him while he avoids nasty paperwork accounting.