There's an important difference between asking "Is it ok..." (to abort/take property/whatever else) and asking "Is it theft...".
In essence, the argument is using our negative feelings about "theft" to distract us from the relevant question of "is it ok/good?".
It's a technique that works for any category of things where there's a spectrum from bad to good and you want to persuade someone that the good instances are actually bad.
To take the abortion example. You ask someone who's undecided on abortion, "Is it okay to abort the moment after birth? What about the moment before birth? An hour before birth? A day? A month? 8 months? The day after conception?"
Phrased this way, the answers are likely to change from "no" to "yes" at some point through the spectrum. The person will likely feel it's okay to kill a single celled zygote with no brain and no mind, but not okay to kill a freshly born baby, and this phrasing makes it easy to acknowledge the difference.
But you can muddy the waters by changing the question from "Is it okay..." to "Is it murder...". After all, if you consider it murder to abort a fetus just before birth, it's not obvious why you wouldn't categorized aborting younger fetuses as murder.
One reasonably answer might be that you could consider killing a fetus/embryo at any age to be murder, but that still doesn't make it wrong to kill at early stages.
A technically correct answer would be it's not murder because our legal definition of murder excludes abortion.
However, people presented with this argument often won't say either of these things, and that's why this technique is effective. It obfuscates the question we're actually interested in ("is it okay?") by breaking it into two parts. It's trying to convince you that 1) killing a fetus is murder, 2) murder is not okay, therefore killing a fetus is not okay. This is an obvious fallacy. If you change what's considered murder to include something that is okay, then you've invalidated 2). However, these last steps of the argument are usually subconscious and people miss the fallacy.
The taxation argument has the same problem. Sure, you can call taxation "theft" if you want, but that does nothing to explain why it's bad. Or we can stick to the official definition of theft, which excludes tax. Either way, they've failed to explain why tax is bad.