But humans are made of meat, and words and phrases have connotations. There's a difference in the perception (both to the subject and society) between those two options.
"Cannot exceed" makes it pretty clear that it's a maximum bound, and doesn't imply that the actual number will be any particular distance between zero and the maximum. "Up to" leads the reader to assume that the likely sentence is close to the stated amount.
Honestly, I think most readers will be more familiar with how “up to” doesn’t mean that it is likely to be close than with the meaning of “cannot exceed”, from experience (as “up to” is regularly used in this way commercially), but, yes, unfortunately given only one figure, even if clearly marked as an upper bound, people who aren’t actively critically reading are likely to fixate on it as if it was a prediction of the likely result rather than a bound.
Press releases aren’t designed to scare the subject of the investigation, especially not press releases announcing a plea agreement that has already been reached.