If a MacBook 2017 pro battery expands and pops open the case should it not be repaired for free?

Apple does not consider a swollen battery a safety issue. It is just a defective battery. I personally consider the swollen batteries a bit dangerous as it can release toxic chemicals if it begins leaking. So far I've never seen a swollen battery leak and I've seen a lot of swollen batteries. Some have been so swollen that I've been afraid to touch them for fear it would pop.

Apple does have several free repair programs for various battery issues including some secret free repair programs. The free battery replacement programs only cover the laptop for up to three years since the date of purchase. If your laptop is less than three years old, then the repair should be covered (you may need to provide proof of purchase to confirm the date if Apple does not have it on file). If the laptop is more than three years old, then you will need to pay to have the battery replaced.

Of course Apple has the final say on any coverage options. Your only recourse is to provide feedback or to consult your local consumer laws.

IIRC if the laptop is sent to an Apple repair depot by the Apple Store or AASP, then a regular battery replacement cost is $129. I'm not sure if there could be any extra costs for the Bottom Case being bent.

FYI, here is a current list of the publicly acknowledged free repair programs for various Apple products (some such as your laptop may have multiple repair programs available).

https://support.apple.com/exchange_repair

Hi,

15-inch MacBook Pro battery recall program.

https://support.apple.com/15-inch-macbook-pro-battery-recall

You'd better contact Apple support or make Genius Bar reservation or take your MacBook Pro to Apple Authorized Service Provider to replace the battery for free.