That would scare me as advertiser. I don't want my ads on certain sub-reddits.
We limit the subreddits your ads can show on outside of the targeted subreddit to ones that are similarly "brand safe" to the ones you are targeting. (e.g. if you are explicitly targeting only NSFW subs your ad will show up on other NSFW subs, but if you're not targeting NSFW subs your ad will never show up on a NSFW sub). You are also able to exclude any specific subreddits you don't want to show up on.
The whole point of marketing is to build an association between a context and a brand - if you can't control what context in which your ad appears then the platform becomes nearly useless.
Worse than useless, actually, since you could be paying to build association with context you that damage your brand. I would be outraged if my ad for /r/luxury ran on /r/couponing, regardless of whether you consider it to be NSFW.
The ability to target specific subs has multiple benefits:
- the advertiser knows precisely where the ad's appearing. Useful for them and their notion of where it'll be most useful, and indeed, knowing it won't be running anywhere else. Reddit appears to be deliberately diluting this benefit.
- the reader had previously been assured of ads relevant to the sub. As has been noted elsewhere, it's not always welcome for ads from professionally relevant subs to bleed outside the work day. Then there are subs which people might not want ads targeted for them appearing elsewhere - I'm not entirely confident someone who'd visited a sub on the basis of tentatively exploring a possible element of their sexuality, for example, might want that highlighted the next day when visiting some other sub entirely.