Ask HN: Why does Google allow extensions using ExtPay and similar

I noticed a HN post promoting an extension which seems to modify your cursor to sparkle. There are several extensions like this. So I downloaded (not installed) it and had a very quick look at it and noticed it installs a script called ExtPay.js (https://github.com/Glench/ExtPay)

This looks very shady to me so I wonder: Why would Google allow such scripts in the first place?