I've always found this take on in-product purchases and subscriptions weird.

You are right that they're allowed to do whatever they want, but...this is just my personal take...the value proposition for Nitro is pretty low as it is. Trying to get more than a subscription from me is a bit of a turn off and makes me want to reach for the cancel subscription button (actually, my subscription is currently in this state through to the renewal date because of the nags about paid borders and stuff).

I do this with this and also other services that want to upsell beyond a premium or support the platform experience. If I'm already supporting the platform, the first time I'm asked to support it more is when I cancel the subscription. Then they have the uphill battle of convincing me to resubscribe in the future.

Stated differently, if they don't remind me I'm subscribed, I'd just keep paying. If they remind me by asking me to pay for things over and above a subscription, I'm suddenly trying to find the true value of the new thing, and also in the next subscription payment. If I can't decide within a few minutes, I always just hit cancel.