Yes, they shouldn't be doing that. It's equivalent to signature content, which should be within the signature area only. If it doesn't fit or isn't allowed, then it isn't allowed.
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What they are doing is highly annoying. It is much like the "bitcointalk for android" signature that some people include as part of their posts (I see less of that recently, but it used to be common), and is essentially advertising their website in each of their posts