Signing up for no deposit bonuses is a big marketing trick with most casinos.
I see no issues with people doing that to try to get lucky.
Very few get to meet the wager and they can now forever spam you with emails to try to get you to deposit further, which is far more valuable then the odd customer winning.
We are lucky that these days you have to give permission which ways you allow them to contact you about marketing, promotion etc... related stuff. If you tick your consents as NO to all possible methods they suggest and keep receiving emails, they have broken regulations and you can get them to trouble.
Can't see anymore (10 years ago we were living in different time) that these no deposit sign sign up bonuses wold bring much value to casinos as online casino existing is no news to any gown up person anymore which would need to get people there to understand what it is. Guess we are not fancying too much 10 spins in Starburst, not giving you really any picture if casino is nice or not (except if you already realize that site is lagging etc...), you can find it out without them. For casinos it help to use these every year smaller bonus budgets to players who actually make at least that one deposit instead of people register just for freebies and never even tempt to deposit. Then some of human minds have weird perversions to create multiple accounts one after other to places where you get something for free even they know that in case of winning, they don't get paid but still doing it to get few spins and from all these multiple accounts and no depositing customers casino don't get anything but possible waste of time and money as every spin raise bonus costs and need to paid to game provider.
Not saying that there is anything wrong to use only welcome freebies but that for casinos these start to have very small or non benefit in offering them. Which can be seen more and more in sign up bonuses don't have freebies, as a player really prefer bonuses for depositors rather than all for free as these freebies can be added to depositors bonuses