They're not allowed to offer that deal up front, they aren't allowed to talk about a major league contract at all. All they can offer is a minor league deal with the maxed out signing bonus.
Section H in it’s entirety of Attachment 46 to the 2017-21 MLB/MLBPA Basic Agreement speak to all of this.
http://www.mlbplayers.com/pdf9/5450407.pdf
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They aren't even allowed to guarantee Otani that he'll be added to the 40 man roster, let alone specify a date in which it'll occur.