I don't think the terms of Google's coupon are that 'you can only use this credit if you intend to spend more than $100 long term advertising on our platform'.
If it were a matter of 1 user creating multiple accounts then they'd have a legitimate reason for closing the account. However, it doesn't sound like that's what's happening. It sounds like they're punishing people for using their coupon in a way they hadn't intended.
What's the difference between 1 user and multiple users creating accounts to advertise the same URL with free coupons from their perspective? It looks very fraudulent and appears to be cheating their system. It seems reasonable to ban such behavior when it's cheating.
Does Google let one account use more than one coupon? Certainly the terms of the coupon say one per account (I have read that because I did want to use more than one).
If it were a matter of 1 user creating multiple accounts then they'd have a legitimate reason for closing the account. However, it doesn't sound like that's what's happening. It sounds like they're punishing people for using their coupon in a way they hadn't intended.