Agree with this 100%. You occasionally get people in the MB forums or FB groups who have either not understood how the system works or who are placing bets that are not properly matched, ie. gambling. These people will generally have the error of their ways pointed out to them very swiftly.
The vast majority of matched betting offers, when done properly, are risk-free, as long as you've understood the process (spend a LOT of time watching the videos and reading the guides before you start) and you're careful not to make careless mistakes.
As others have said, you know how much profit you're going to make before you place the bets, and that profit is the same regardless of the result of the event - so pretty much by definition that' is not gambling, since the outcome is already known.
The casino offers are a bit closer to gambling, since they rely on long-term value over many many offers, and you can lose significant amounts of money while waiting for the overall outcome to match the mathematical prediction. But again there are risk-free casino offers: lots of sites have daily offers of free spins or pick a box type games where you will generally not win anything or just a few pence, but can occasionally win large or even very large amounts - I once won nearly £2k off five free spins, for eg.