Amazon has long been subject to speculation of a move into bricks and mortar.
is awkwardly worded. The intended sense of the sentence is that people have been speculating for a long time about whether Amazon will open brick-and-mortar (that is, real-world) stores in addition to maintaining its online marketplace. To convey that idea clearly, you might put it this way:
Amazon has long been the subject of speculation about [or regarding] its possible expansion into brick-and-mortar stores.
Admittedly, if you're the subject of speculation, you must in some sense be subject (that is, susceptible) to speculation. But the crux of the sentence that the poster cites is not that Amazon is (and for a long time has been) peculiarly subject to such speculation; it's that people are (and for a long time have been) making its rumored plans for real-world retail expansion the subject of speculation. That being the case, the writer did a poor job of expressing the main point.
The writer's choice of "subject to" in place of "the subject of" is not entirely explicable, but I suspect that it may have been influenced by a desire to avoid repeating the preposition of in the phrase
the subject of speculation of a move
If so, the writer's fastidiousness on this point was a mistake because the phrase "speculation of a move" is at least as ill suited to the sentence as "subject to" is. When discussing swirling rumors or rampant speculation, English-language writers today are considerably more likely to use the phrase "speculation about a [subject]" (red line below) than to use the phrase "speculation of a [subject]" (blue line below), as this Ngram chart for the period 1900–2005 suggests:
Since the early 1970s, "speculation about a" shows up as being more common than "speculation of a" even though the latter phrase collects matches of the forms "speculation of a [person speculating]" (for example, "speculation of a later editor") and "speculation of a [type or kind]"(for example, "speculation of a scientific character") as well as matches of the form "speculation of a [thing being speculated about]" (as in "speculation of a mutator phenotype"). In contrast, "speculation about a" much more narrowly collects matches of the form "speculation about a [thing being speculated about]" 9as in "speculation about a presidential bid").
Of course, speculation about why a writer makes poor word choices is just speculation. The key point, in my view, is that "speculation about a move" is something that Amazon was more meaningfully "the subject of" than "subject to."