While people are saying legally this can be done or that they are not questioning they can legally make that change. For those of us in the UK the Consumer Rights Act 2015 dose question the legality of this change, at no point until now (and even now its still not listed on the store page) has the Steam version of the game been described as actually requiring Steam to run, this means the game no longer fits its description (system requirements as that is what this is would be part of its description) and as the law states it is the description at the time you were supplied with and not at the time of modification any changes to the description or contract when this is applied are irrelevant to any legal claim.
Side note, if as I saw someone mention because of ESO and ping times increase it means the change will also affect the quality of the game and in my case it may no longer be fit for purpose as I got FF14 to play with my friends on a US server while I am in the UK.
(The term "trader" in this case would mean both Square and Steam)
Code:
40
Quality, fitness and description of content supplied subject to modifications
(1)Where under a contract a trader supplies digital content to a consumer subject to the right of the trader or a third party to modify the digital content, the following provisions apply in relation to the digital content as modified as they apply in relation to the digital content as supplied under the contract—
(a)section 34 (quality);
(b)section 35 (fitness for a particular purpose);
(c)section 36 (description).
(2)Subsection (1)(c) does not prevent the trader from improving the features of, or adding new features to, the digital content, as long as—
(a)the digital content continues to match the description of it given by the trader to the consumer, and
(b)the digital content continues to conform to the information provided by the trader as mentioned in subsection (3) of section 36, subject to any change to that information that has been agreed in accordance with subsection (4) of that section.
(3)A claim on the grounds that digital content does not conform to a term described in any of the sections listed in subsection (1) as applied by that subsection is to be treated as arising at the time when the digital content was supplied under the contract and not the time when it is modified.