For essentially all companies whose customers are paying consumers:
The technical staff all talk about the dangers of Friday evening deployments.
The company has ignored/overridden these warnings in the past, and sometimes experienced catastrophic failures because of it.
STILL, sometimes the marketing staff will have weekend promotions that are considered more important than these concerns.
In particular, a weekend promotion will be implemented, tested on Wednesday, and ready to go. And only after it's active, is a bug found in it on Friday at 5pm. And the promo is more important than the risk of failure.
I'm not saying this as a "bitter techie"... I'm just explaining that this is how it happens.
I think that the "it's more important" argument is often correct. Not running the promo costs the company $X of opportunity cost, and X is often very well known, from past promotions. Balance "losing $X" against "well, there's only a small chance of something going wrong", and that's why the risk is often taken.
Take the risk enough times, and eventually something goes wrong.
