Merit sources should do their best to keep their available source merit close to zero (ie spend all their source merit), and doing so is not always easy if they do not read a lot of good posts. This means they may need to send a lot of merit per post.
No punishment to merit sources if they don't use all sourced smerit so they should not force themselves to send sourced smerit more than a post deserves to get.
There is no punishment, but theymos has encouraged the merit sources to spend all their source merit. If I do not spend all my source merit, it will go to waste, so I might as well give more merit on a per post basis so all my source merit is depleted. Theymos has said in the past that it is better to give one post 50 merit if you can only find one merit-worthy post in a month and have 50 source merit rather than let your source merit go to waste.