People who spam post with an advertisement in their signature area are giving something of value (advertising) to the company they are advertising for. If the amount that posters (advertisers) are getting paid is not cost effective to the advertisers then advertisers will offer a lower payment for such advertisements, or withdraw offers to have them advertise at all. The fact that the price of BTC has gone up substantially in recent weeks means that a company who was considering to lower their advertising rates can do so while still maintaining as high of rates when measured in USD.
No one is forcing anyone to accept low BTC rates in exchange for advertising and if someone is offering low rates then you can ask for something higher or you can decline to advertise for those who offer low rates.
I doubt that in a week's time these companies have realized some direct impact of effectiveness on the advertising and cost benefit between how much business they bring in and how much they want to payout.
In a weeks time? No, however companies may have been looking for ways to cut advertising costs, or they may have been on the verge of "pulling the trigger" of lowering rates, and the recent runup in the USD/BTC price may have simply been a good time to do so because their advertisers would be receiving a similar amount in terms of USD.
These companies are ONLY worried about how much they'll be paying out in terms of fiat value, and they don't want to pay that much. It's smart from a business sense, but it doesn't make any of their campaign posters more loyal to their campaign.
True, however they mostly have revenue denominated in BTC, therefore it would be prudent for them to attempt to have their expenses denominated in BTC. Not only that but when the price of BTC increases, people holding BTC often are more willing to spend their BTC so when BTC increases is exactly when companies are going to want to have the most advertising.
It also hurts the bitcoin community/economy because it shows that the's companies are not committed to doing business in Bitcoin...they're still just focused on how much fiat they can get for their bitcoin.
The signature campaign rates as of ~2 weeks ago were roughly in line as to what they were ~1.5 years ago when BTC was trading at roughly US$700-800. IMO most of the campaigns that lowered their rates were highly associated with spam, and it would probably be a fairly good thing if a good number of the people advertising for them stopped doing so.
It is also important to note that not all campaigns have lowered their rates.
I, too, have not cashed out my campaign earnings. I think of it only as bitcoin and I'm disappointed that I now need to work nearly twice as hard to earn the same bitcoin - particularly when recent publicity has probably brought more people to this forum to see the ads in my signature.
I would suggest dropping your campaign (they are associated with a lot of spam anyway) and joining one that is higher paying.