As a preface, every company must have a team of lawyers who interpret the rules and who know the practice of the Treasury Department, so that the company doesn't violate the rules and end up in big trouble. A company has to decide whether some action is worth the risk (meaning, knowing what is it worth to them, and what are the risks). There is a long FAQ from Treasury which tries to interpret the rules. A simple answer to the question is that a company might decide that the risk is too great, even if many or most specialists in the legal issues don't agree that there is an assumed risk.
31 CFR 560.204 states:
Except as otherwise authorized pursuant to this part, and
notwithstanding any contract entered into or any license or permit
granted prior to May 7, 1995, the exportation, reexportation, sale, or
supply, directly or indirectly, from the United States, or by a United
States person, wherever located, of any goods, technology, or services
to Iran or the Government of Iran is prohibited, including the
exportation, reexportation, sale, or supply of any goods, technology,
or services to a person in a third country undertaken with knowledge
or reason to know that:
(a) Such goods, technology, or services are intended specifically for
supply, transshipment, or reexportation, directly or indirectly, to
Iran or the Government of Iran; or
(b) Such goods, technology, or services are intended specifically for
use in the production of, for commingling with, or for incorporation
into goods, technology, or services to be directly or indirectly
supplied, transshipped, or reexported exclusively or predominantly to
Iran or the Government of Iran.
Reducing the verbiage, exportation of technology to Iran is prohibited. It is thus not a stretch to think that exporting technology to Iran is prohibited for a US person or company.
It is possible to obtain a license to export to or from Iran, see 31 CFR 560.501 ff.. It is completely not clear what it takes to get a license: I suggests that differences between US companies may relate to whether a particular company has a license to export. There are also prohibitions targeted at specific individuals and organizations. Transactions over the internet can be risky, because a website cannot compel a user to truthfully identify themselves, and "E.O. 13599 requires U.S. persons to block all property and interests in property of the Government of Iran, unless otherwise exempt or authorized by OFAC", and absolute blocking is an extreme form of due diligence.
The political questions as to the wisdom of the sanctions are appropriate for Politics SE.