He did get it in writing, the text messages.
Of course contracts with amendments protect all parties, but it's just not done even close to 100% of the time, even with the most sophisticated companies.
If you're a company like Target, you have general supply agreements (that establish Ts & Cs and pricing), but they are constantly doing business and making orders via email. Should they have a a signed agreement for every order? Probably. Is it feasible? Probably not. The truth is, even if the other side welches on their bet, you have the opportunity to sue them (like these kids are here) or simply cutting them off from future work. It's a cost/benefit analysis where you weigh the cost of legal fees.