If the USPTO could control the patents it gives out so the rate they're overturned upon challenge is low, then it makes sense to force violators to prove the patent is invalid. But because they're seemingly willing to give out patents for anything and the rate they're overturned, it makes more sense to shift the burden onto the patent applicant to take reasonable steps to make sure his patent is ironclad and will not be overturned. If the patent applicant's confidence in his own patent is so low he isn't sure it won't be overturned upon a detailed review, then that's a pretty good indication the idea isn't really worthy of a patent in the first place.
This also has the effect of making pure IP companies a high-risk business. If all you do is license patents and one of your main patents gets overturned, it could bankrupt you. But if you're actually using the patent to make stuff, then you'll have an alternate revenue stream which will allow you to survive having to pay back the licensing fees.
There is a drawback in that companies may be more willing to license specious patents, in hopes that someone else will go through the expense of fighting it. If someone else fights it and wins, you get your money back, so why should you fight it? On the patent's holder's side, this creates a multi-year potential liability in the accounting books even if you have a valid patent. A sunset period of a few years after which you can't recover licensing fees (or a graduated return period, so after say 3 years you have to pay back 50%, after 5 years 25%, after 7 years you can keep it all) would address both problems.