The new "generation" of AC adapters do not have this problem. The old ones were just a transformer and rectifier, with no regulation circuit on the output. If the input went up, the output went up by almost exactly the same percentage.
New AC adapters are miniature switching power supplies, with true regulated outputs, transient protection, etc. They are much more efficient, or "green", which is the driver these days. One chip vendor has price data showing that if you are building more than 1,000 units per day, the new switching circuit is cheaper than the old transformer type. This cost savings has NOT made it into the end user market. In round numbers, a single output AC adapter is around $10 for the old kind, and around $20 for a new switcher type.
By the way, the worldwide market for AC adapters is over 2 billion units per year, or about 10 million units per business day. Impressive.