Late last week, some of the biggest companies in the U.S. submitted a "friend of the court" petition supporting marriage equality to the Supreme Court--with a few notable absences. 

The High Court is expected to rule on the legality of same-sex marriage by the summer. The petition was signed by nearly 400 businesses, including technology leaders Amazon, Apple, Facebook, Google, and Microsoft. They joined such non-tech stalwarts as Coca-Cola, DuPont, and Johnson & Johnson. But notably absent were prominent tech startups that have been active in other high-stakes policy debates, as well as some other tech companies that have been embroiled in diversity controversies.

After taking heat for gender insensitivity, on Tuesday Uber, one of the biggest startups in the world, announced it would hire one million female drivers by 2020. Yet on the issue of same-sex marriage, the company remains mum. You won't find its biggest competitor, Lyft, on the petition either, or the newly public Etsy, which went to the mat in its efforts to get the Federal Communications Commission to support net neutrality. Nor will you find another likely candidate, Mozilla, whose chief executive stepped down when it was discovered he'd supported a California proposition that overturned same-sex marriage in the state in 2008.

But Susan Baker Manning, lead counsel on the brief submitted by law firm Morgan Lewis & Bockius of New York, says the names on the petition expanded on a prior list and shouldn't necessarily be taken as an example of which businesses support marriage equality, and which do not.

"The expanded list of [friends of the court] shows the interest of the business community in this issue, and their desire to make their unique business perspective heard by the court," Baker Manning says.

More specifically, Baker Manning says the signing companies wanted to express to the High Court how lack of marriage equality negatively affects their businesses. The uneven patchwork of laws regarding marriage equality can make it difficult to retain and attract top talent, if for example, you ask a married LGBT person to move to a state that does not support marriage equality.

"Essentially you're asking employees to tear up their marriage licenses when you ask them to transfer to or hire them in a jurisdiction where their relationships are not recognized," Baker Manning says.

It becomes an administrative headache for businesses as well, where benefits and tax obligations might change according to a state's recognition of same-sex marriage, Baker Manning says.

A High Court ruling paved the way for federal recognition of same-sex marriage in 2013. And currently, LGBT people can marry in 37 states, although that right is being contested on other grounds in a new Supreme Court case called Obergefell v. Hodges. Some states are also passing laws that would make it legal for businesses to discriminate against LGBT people, on religious grounds.

Akamai Technologies, a content delivery and cloud services company based in Cambridge, Massachusetts, signed the friends brief because supporting marriage equality is consistent with its core values of encouraging a diverse workplace, Robert Morton, a company spokesman said.

"Akamai fundamentally believes the workplace can suffer when federal law forces an employer to treat some married employees different from others," Morton said. Akamai has 5,000 employees around the world, and offices around the country.

In 2013, about 280 companies signed an amicus brief in support of Windsor v. the United States. And in this go around, it's likely even more companies would have signed on had their been more time.

Lyft and Uber did not return requests for comment, and Etsy declined to comment. For its part, a Mozilla spokeswoman said it was not asked to join the brief, but the company made its commitment to same-sex marriage apparent in a blog post from last spring, when Brendan Eich stepped down as CEO. Similarly, an Indiegogo spokesman said that Indiegogo would have signed the brief, had it been asked to do so.

"Indiegogo is an open platform that strives to have a diverse workplace, and a big part of that is the LGBT community," the spokesman said. "We absolutely would have signed the brief."