I always thought it was cos Roth was pissed at Mike for even daring to ask that question like it was business and what gave him the right to question a man like himself, the great Hyman Roth.
The way Roth glances at Johnny Ola always seemed to re-inforce that for me, he looks at him as if to say 'can you believe this little punk'
The speech he goes on to give Mike

There was this kid I grew up with -- he was younger than me. Sorta looked up to me -- you know. We did our first work together -- worked our way out of the street. Things were good, we made the most of it. During Prohibition -- we ran molasses into Canada -- made a fortune -- you father, too. As much as anyone, I loved him -- and trusted him. Later on he had an idea -- to build a city out of a desert stop-over for GI's on the way to the West Coast. That kid's name was Moe Green -- and the city he invented was Las Vegas. This was a great man -- a man of vision and guts. And there isn't even a plaque -- or a signpost -- or a statue of him in that town! Someone put a bullet through his eye. No one knows who gave the order -- when I heard it, I wasn't angry; I knew Moe -- I knew he was head-strong, talking loud, saying stupid things. So when he turned up dead -- I let it go. And I said to myself, this is the business we've chosen -- I didn't ask who gave the order -- because it had nothing to do with business

seems to me like a 'how dare you question me?' speech to Mike!! : only my opinion, for what it's worth But I am gonna have to rethink this now....

FH