Your problem started when you hired a gardener to do plumbing work.
John
Amen.
But more specifically, I would point out to you that although you are now in violation of Federal law, and the chances that you are going to be cited for it are minimal, what is certainly true is that you will be the first victim of this violation.
That is, the law is designed to protect the public water system as a whole, i.e. the public at large, from ingesting pesticides, fertilizer, etc., from your garden hoses or sprinkler system.
However, when that gallon of Roundup or Atrazine or Chlorpyrifos gets sucked into your garden hose and back through your home water system towards the public water supply, what water system do you suppose it's going to end up in first? You got it, yours. Your shower, your dishwasher, your kitchen sink, your icemaker. "Mom, can I have a glass of Iced Roundup?" Yum.
So, tell the hack moron to put the backflow preventers back on, or, better yet, get a local plumber to fix it right for you.