It doesn't shorten their life spans, and if it did, first I heard of it in the 20 years I've been keeping fish.. A lot of breeders who use outdoor containers, and people who live in Thailand, etc where it gets really hot would not have as much success if that were the case.. not to mention natural environment it gets that warm.
High temps won't keep everything at bay, fungus thrives in higher temps.
It is also hard to find smaller heaters that will go that high, remember most people keep their bettas in 1-5 gallons and use 50w and lower heaters. Some are preset and some go higher.. varies on brands.
What is the most important factor in keeping fish healthy and alive is water quality. You can have their tank at 90 degrees, but if you aren't doing proper water changes they will still become ill, regardless of temp. So water quality is top priority. Followed second by food quality and variety - then would be temp.
A betta can live in cooler waters as low as 68* and still be fine - may be a bit more lethargic, but can live in it for a good amount of time before it affects them.
Ideally, 76 - 80 is what is recommended if the betta shares a tank with other tropical fish (which is common). And depending upon some species of plants, they do better in the mid 70s to low 80s.