Really any company that is this stupid you probably don't want to work for unless you are overpaid.
This is a huge red flag - stay away, especially if they insist on this after you've explained that you want to discuss salary first.
In the best case, the offer is what you were expecting or better. You would have taken it if they had disclosed it up front too so they aren't any better off.
In the worst case, it is lower than you would have accepted. In this case, you could back out of the deal or accept it and start looking for a new job immediately. Both of these will just waste the company's time and money.
Are you working with an external recruiter by any chance? They do stand to gain from strong-arming you into accepting an offer. If this is the case I would try to contact the hiring manager and ask them about this policy.
Also is this company in the US because I don't think any lawyer would approve of this.
"If we offer you this position and you like our offer, are you going to accept it?"
If so, that does not seem unreasonable to me.
Perhaps they have been burned by tire kickers in the past who were only looking for a way to increase their current salary with a competitive offer with no intention of ever taking the position.