While that guy's response to you was completely unacceptable, you should know that there are several reasons not to use the wiper fluid while moving: it obstructs your view of the road for a period of time, in most cases you can and should use the wiper fluid before you start driving (I realize this is not possible if the windshield gets dirty in transit), and it's inconsiderate to other drivers - you don't have to be tailgating someone to be hit with their dirty soap spray and in general it's best not to piss people off on the road if you can avoid it.
It may be unreasonable to ask someone to pull off a highway to use their wiper fluid every time they hit a bug, but it isn't unreasonable to ask someone to consider waiting until there is some free space behind them and it isn't unreasonable to ask them to wait until they are at a stop sign or stop light (if one is coming up).