Why did they begin using *fuck* in Season 7 (2017)?

Here is a good article that explains why shows in general have become more comfortable with the usage of that specific expletive.

just a note, use of swear words in quotes have been censored, some people are not comfortable with them, and as censoring swear words did not alter the meaning of this article, I censored it for those not comfortable with them. Aside from that, it is a direct quote

use of f*** in shows

As the article explains, it has more to do with the actual network and the advertisers those networks use than it does a specific show. It mentions several networks, including the network Suits is on, USA. In answer to your question about whether any sort of legality is involved, here is a direct quote:

"Previously, swearing on SyFy and USA stuck to the guidelines laid out by the Federal Communications Commission, but as a basic cable channel, their Standards and Practices division was not actually beholden to follow those rules strictly. Instead, they answer to their advertisers, who traditionally don’t like it when things get controversial on television programming, and that includes salty language"

Here is another quote that explains why they have begun to use the word.

"The only thing holding back basic cable networks from using what is considered to be more vulgar language is their advertisers. To keep things clean, they usually dip the audio of either the “f” or the “k” whenever “f***”(quote altered for censorship) is said in an episode. But according to Buzzfeed, USA and SyFy have worked that all out because their stance now is “when language — ‘f***’(quote altered for censorship) specifically — is deemed important to the style or plot of a show, Syfy and USA now allow it.” That means the episode gets slapped with a TV-MA rating so audiences know it’s intended for mature audiences only."