Because once they have your email and can link it to your identity via your purchase details they’re going to sell that list to some marketer sleazeball and you’ll get spam from other sources until the end of time?
So … ops normal?
If you get more than "insignificantly little" spam in your inbox, you are using the wrong mail provider.
My email address is on every spam list under the sun. I get 600 spam messages per day[0], but only a few per week hit my inbox.
[0] It was 600/day before I made a small change to my mail configuration. Now it's only about 50/day which is few enough that, every month or so, I actually check for false positives. I occasionally see a low-value marketing list message that isn't technically spam in the sense of being entirely unsolicited, but content-wise it's not differentiable. Zero legitimate personal messages. I can live with this.