Is this legal?

On a competition last Saturday, in the quarter finals, the opposing team “accidentally “ put our mobile goal in their 10pt zone. We have read the rules and the rule that concerns this doesn’t mention anything about accidental vs intentional. Also that mobile goal would have been match affecting. So should the opposing alliance have been DQed?

Yes. We had a similar problem where the opposing alliance tipped a mobile goal in their 5 point zone and then I pushed it into their 10 point zone in order to untip it (we can pick up out of 10 point zone) and got it out, but the refs started yelling about us being in their 10 point zone so I decided to just put it down somewhere else and just no use it so we wouldn’t get DQed. After the match we talked to the refs and they confirmed that it would have been ok to score on it since we didn’t disrupt the opposing alliance by getting it.

Here’s the game manual about handling opponents mobile goals:

<SG11> Robots may not put a Mobile Goal in an opposite Goal Zone. (i.e. A Robot, regardless of
Alliance, cannot put a red Mobile Goal in a blue Goal Zone or a blue Mobile Goal in a red Goal Zone.)
Minor violations of this rule that do not affect the Match will result in a warning. Match Affecting
offenses will result in a Disqualification. Teams that receive multiple warnings may also receive a
Disqualification at the Head Referee’s discretion.

Here is more from the Official Q&A summary:

Interaction with Opposing Mobile Goals
Game Manual Reference: <SG6>
Grasping, grappling, or attaching to Field Elements or opposing Mobile Goals is illegal. Other
synonyms for illegal actions would include latching onto, anchoring to, gripping, pinching, clamping,
lifting, or carrying. However, pushing, encompassing, tipping, covering, or otherwise passively
manipulating the Mobile Goals (without “grasping”) is legal.

I would have difficulty seeing how an opposing team could get a mobile goal into their own 10 point zone without lifting or carrying it. It would take more than legally shoving it around to do so (and accidentally vs intentional in the 5 pt zone is difficult to determine, and not too difficult for the opponent to recover anyway), unless they tipped it like @Silicon mentioned, but that would be risking DQ.