In my opinion bonus abusing is something that doesn't respect TOS while instead matched betting does it.
They are two different thing, bonus abusing is obviously technically illegal, that once observed and detected the account will be subject for termination, worst to lock its funds too. While matched betting is technically on the other way around. But still depends on the terms of the free bets, if theres no terms that prohibits such doings then you can match bet.
Sometimes you need to read and understand the rules and terms for free bet as well not just the tos of the games/website.
at least the player is spending his own money in matched betting. for me, this is fine as long as you are following the rules of the site when it comes to betting. but with bonus abuse, you are literally going up against the site's ToS. and yes, as said by TelolettOm, form of cheating.
with matched betting, you are just making sure that you won't lose a lot with your bet money. in my opinion, so long you are not breaking the rules, you are more than happy to bet what you want.