To be fair, there's a difference between "you shouldn't do this" and "you shouldn't be allowed to do this". At least in this letter, the senators are not explicitly trying or threatening to ban anything.
'Just asking' is a lot worse than following the legislative process. They are trying to use their influence to accomplish an effectively legislative outcome without oversight. This creates a dangerous gray area between law and intimidation. It's similar to when Visa, Mastercard, etc. were 'just asked' to stop dealing with Wikileaks. In fact, it is common for dictators and totalitarian regimes to rule through 'just asking' instead of law. Not a good precedent.
I was unclear. "You" was referring to Apple, not the customers. They're saying you shouldn't be making this available, not you shouldn't be allowed to make this available. Not even your customers shouldn't be allowed to use this, and I feel there's a distinction between something being generally unavailable and something being illegal.
But you're right that if Apple makes it unavailable, it will have a similar effect as if their customers weren't allowed to use it.