A good question, DC.

I always look to the book to provide explanations when possible, but in this case what Puzo writes only confuses the issue further.

While discussing the upcoming meeting with Sollozzo, Sonny says to Tom

"He (Sollozzo) wants us to send Mike to meet him to hear his proposition. The negotiator guarantees Mike's safety. Sollozzo doesn't ask us to guarantee his safety, He knows he can't ask for that. No point.(italics mine)

Why couldn't he? Why was there "no point" in him asking?