As long as I have been online and working get paid sites, it has been rare to see many with 90 day or more inactivity terms. Usually, it starts at 15 days upwards.
Darkstar is right, the Terms of Service/FAQ areas on each site usually spells out how long you can be "absent" without losing your account/cash. It's a very good idea to read over these two areas, and if you have questions, use the "contact" button. Most program owners will answer.
Thing is, with so many traffic exchanges and a few PTC/Bux/PTR sites out there forcing you to upgrade in order to be paid, it's a MUST to read the Terms closely. Especially if they pay through PayPal. It's against PayPal's Acceptable Use Policy, and PP *might* freeze that site's account.
There are times a person gets hit with some major event they couldn't help (i.e., illness, death of loved one, accident, computer blowing up, home tossed by nature, etc.), however, there are computers in many libraries and some book stores, hotels, cafes one can always quickly sign on, go to the site's contact area and send a short note to the Admin. It only takes a little bit to do so, and most likely would save you loss of account, cash, referrals.
It's concerning about keeping the cash already earned, but the Admin has no way of knowing you would actually receive it, so that cash usually goes back into the "kitty". It's a Catch 22, but there are good reasons this happens.
ClixSense will most definitely pay if you are active. For the most part, they are keeping their end of the Terms.
I do think the suggestion made by seaeagle is a good one, though. An email to people nearing the 90 day mark might work. Giving the member a slight push to see if they come back. I don't know how the scripts work here, but in a lot of sites, there is an area where you can contact specific people for anything to do with their account via email, and account page.
I do disagree with your comment about CS [sucks]. CS expects you to read the Terms BEFORE you join, if you did not do so, it actually is your fault as you didn't research the site beforehand. And that's something everyone should be doing. Just like forums where admins actually do their jobs correctly...they will ban you if you don't follow the rules. At least at CS, even if you go inactive for a long period of time, you can come back.