Is it because the guns are much more likely to be stolen nowadays, or is there some law prohibiting carrying firearms in the back window of your pickup

Well, it may be illegal someplace, but I have never lived anywhere where it was prohibited by law. But the danger of theft is very serious these days, and you know how law enfrocement goes bananas today about stolen guns! I think a truck rack would NOT qualify as adequately secure, unless you never left the vehicle unattended.

I personally would never subject any of my current guns to such rough transportation. I gave away my truck gun, a totally "naked" Longbranch No. 4 SMLE .303 from WWII. I bought it for $5.00. It would consistently hit a 55-gallon drum at 100 yards with issue sights and the old Dominion 215-grain round-nose factory load. I figured any moose I encountered at ranges under 25 yards with this rifle was dead meat!