What a load of rubbish. There is no legitimate reason for people to have multiple accounts on pretty much any service or site, except for a genuine mistake where someone may have forgotten their old account but would still immediately forfeit any advantages gained from the duplicate account. There is even less reason to accept multiple accounts when it comes to gaining access to "free money" that is awarded via welcome offers or other special promotions. The casino is a for-profit company, their calculations for these special offers are all based around a single customer redeeming them with a single account. Once people start abusing these offers by creating multiple accounts then they start to lose money and the whole incentive scheme is undermined, they either deal with the abusers or other genuine customers lose when the offers are pulled. All this is stated in the terms of service, the contract, that a player enters into and agrees with in order to play there. You're talking about hypothetical situations where people don't redeem the deposit bonus, but this person was taking advantages of such promotions.
That is just not rubbish but my own opinion, I just hate that some people may not take advantage of the deposit offer, they will register on a gambling site, their deposit will not be suspended but their withdrawal would be suspended, no one is againsting that.
But if their withdrawal is suspended because of multiple accounts, I then meant that it is not bad for the gambling site to still be compassionate and check if the person have abused the 2 accounts like through deposit bonus offer. Although that is not yet related to what OP said but my opinion which may lead to further discussion.
But my advice will just be that people should avoid having multiple accounts on gambling sites, the gambling site is centralized, someone's account can be disabled, withdrawal can be disabled, the reason we should fully comply with the ToS of a gambling site, if not comply, the gambling site still has the legal right to seize the violator's fund on their site. So I can say that you are not wrong.