This is by design. The downside to background pages is they consumed an entire page's worth of runtime resources (and depending on how they're set up, they may do that per foreground tab).
It's a headache early-era iOS developers are familiar with, but this move is basically Chrome team saying "We've watched the community try to implement responsible resource handling, and they suck at it, so we're taking some of their choices away so that the browser can manage resources for the user." Because battery matters.
