That's boilerplate language for pretty much every job description in the United States (which is where I'm guessing you are). It isn't unique to government jobs. Almost everybody that doesn't work under super-strict bureaucratic oversight has to do work that isn't explicitly called out in their job description.
> I live in fear of the day I'm called upon to climb a tower because they need me to do that. My regular job is software development!
Your boss isn't going to come to you saying "go climb a tower or ur fired lol" while pointing to that clause. Tower climbing requires many special considerations like training, permits, insurance, and so forth that you don't have. They will contract that work out or hire somebody to do it specifically at the appropriate rate.
> my position isn't union protected
Last I checked government positions have about half the turnover that private-sector ones do. You're already doing well as far as job security (reorgs, funding SNAFUs, furloughs, etc. excepted).
I can only infer how you came to any of the conclusions that you did in your post but it sounds like you might have uninformed coworkers saying these things. If this is the case then stop listening to them.