It doesn't matter if it's for pot or not. There are rules when on probation. You violate them, you go to jail. Plain and simple. It's just like if someone is on parole. You can't drink or buy alcohol or anything like that. You get caught doing it, you go back to prison. If the rules were you can't buy M&Ms and you buy M&Ms. That's what happens. Whether or not it's weed is irrelevant
It's the same for people that get out on parole. You see it as "it's just pot, that's ridiculous" and it would be for a law abiding citizen without probation or parole. But even though things like weed or alcohol are legal, you are not allowed to have them while on probation or parole. That's just how it works