The National Electrical Code has code explaining how to protect direct-buried broadband communications cables. However, they also provide an exception to that code, that allows cable companies to basically provide the cable no physical protection at all.
In the NEC you'll notice 830.47(C), which provides adequate protection to the buried cable.
National Electrical Code 2017
Chapter 8 Communications Systems
Article 830 Network-powered broadband Communications Systems.
830.47 Underground Network-Powered Broadband Communications Cables Entering Buildings.
(C) Mechanical Protection. Direct-buried cable, conduit, or other raceways shall be installed to meet the minimum cover requirements of Table 830.47(C). In addition, direct-buried cables emerging from the ground shall be protected by enclosures, raceways, or other approved means extending from the minimum cover distance required by Table 830.47(C) below grade to a point at least 2.5 m (8 ft) above finished grade. In no case shall the protection be required to exceed 450 mm (18 in.) below finished grade. Types BMU and BLU direct-buried cables emerging from the ground shall be installed in rigid metal conduit (RMC), intermediate metal conduit (IMC), rigid nonmetallic conduit, or other approved means extending from minimum cover distance required by Table 830.47(C) below grade to the point of enterance.
However, if you continue reading, you'll see the "do whatever you want" exception (that was likely put in the code by a large cable company).
Exception: A low-power network-powered broadband communications circuit that is equipped with a listed fault protection device, appropriate to the network-powered broadband communications cable used, and located on the network side of the network-powered broadband communications cable being protected.
Which basically means, the cable can lay right on top of the ground. Or be pushed just below the grass, as most cable/telecommunication companies do.