It's marketing, really.
Long ago, when LTE was just starting to get rolled out, T-Mobile wasn't yet in a position to launch an LTE network. They were, however, putting the finishing touches on a pretty fast 3G HSPA+ network. So they decided to fudge the terminology and declare that their 3G HSPA+ network was really "4G," even though it didn't meet the official standard for 4G (but sort-of came close, speed-wise).
Well, AT&T got their panties in a huff over that one, so not to be outdone, they too decided to start calling their HSPA+ network "4G". Both carriers modified their Android firmware distributions to label their HSPA networks with the 4G logo.
All of this marketing pressure influenced Apple, who also hadn't (yet) launched an LTE-capable iPhone. So what did they do? You guessed it... Changed the status flag for HSPA+ networks to read "4G" instead of the 3G it initially was. All that happened was the status bar image was changed when Apple rolled out iOS 5.1 back in 2012... Instantly, iPhone 4S users on AT&T were "upgraded" to "4G," without anything else actually changing.
Source:
http://www.pcmag.com/article2/0,2817,2401422,00.asp
Some of us knew exactly what was going on, and called it "FauxG."
Ultimately, the standards body that sets the rules for what 4G is and isn't capitulated, as more carriers and phone vendors started doing the same thing, and the term "4G" was relegated to be a meaningless marketing term.
Verizon and Sprint, sadly (for them), could not really justify jumping on the FauxG bandwagon. They never had HSPA+ on their networks. And the CDMA networks they had deployed were too slow and too feature-limited to justifiably be called 4G.