I somewhat agree that it is linked, but I don't think that signature campaigns necessitate it. I think all that's needed would be stricter moderation.    
Back in what I remember as the peak signature campaign value period, Primedice used to offer upwards of 1 BTC a month, and inputs.io used to pay quite a bit too. (BTC was worth 200smth to 300smth)    
Account seller threads were more frequent IIRC, especially single people selling many many accounts. Account prices were higher too (hero members > 1 BTC I think)

I'd argue that at that point in history, there was more incentive for spamming than there is now.. I think it is just that the standards for removal of posts have lowered, and more and more posts that would have formerly been considered too off-topic or too insubstantial are being allowed now. And you can't really blame the staff, presuming volume of posts is increasing.

Removing signature campaigns would solve the problem. But the existence of signature campaigns and account sales is not the cause, and they have been around from before when the terrible posts came.

Also, I'd argue that the method of micropayments in a signature campaign would actually be good for the forums as it encourages new users (if only they didn't pay per post/have absurd post requirements). I actually remember TF talking about how paying per post was encouraging spam and paying per activity on the inputs.io signature campaign. Straight up removing signature campaigns is going to cut out a lot of the new adopters of Bitcoin who can get their first fraction of a Bitcoin by talking about Bitcoin rather than actually buying it (which can often be a convoluted process not worth going through for small amounts)

So, playing devils advocate, I'd say that signature campaigns don't necessitate spam, it's just alongside lax moderation. Killing the micro-economy of new users who have found usable amounts of BTC only through sig campaigns may be bad for BTC too.