If the other companies suspect Apple is stealing their patents, they can file lawsuits. This is the corporate world. It's not a nice place. At all.
There are a number of companies that have filed patent infringement and trademark lawsuits against Apple.
1. Last week iCloud Communications sued Apple for using "iCloud".
2. 2 years ago, Nokia filed a lawsuit claiming Apple infringed on 7 of their patents. Last week, Apple agreed to pay licensing payments to Nokia. The licensing payments will include a lump sum payment of up to $608 million with an additional $138 million in licensing payments every 3 months.
3. 3 years ago, SP Technologies filed a lawsuit against Apple for infringing on their patent for multitouch.
4. 3 years ago, Klausner Technologies filed a lawsuit against Apple for infringing on their patent for Visual Voicemail.
5. 3 years ago, Minerva filed a lawsuit against Apple for infringing on their patent for a "mobile entertainment and communication device".
6. Last year, Elan Microelectronics filed a lawsuit against Apple for infringing on their patents for multitouch.
7. When Apple originally filed their lawsuit against Samsung, Samsung countersued alleging Apple infringed on their patents in Germany and South Korea and Japan.
8. About the same time Nokia sued Apple, St. Clair Intellectual Property sued Apple for infringing on their patents in the iPhone's camera. This company has also sued many digital camera manufacturers, with both Canon and Sony losing in court, and Fuji, Minolta, Nikon and Olympus avoiding court by signing licensing agreements.
9. Motorola filed a number of suits against Apple for infringing its 18 patents related to W-CDMA, GPRS, 802.11 and antenna design, as well as other smartphone technologies such as wireless e-mail, proximity sensing, application management and location-based services.
10. In 2010, NetAirus Technologies filed a patent infringement lawsuit against Apple for the "concept" of the iPhone since their patent describes a "wireless handset communication system".
I could go on, but that is just from the first 4 pages of search results (excluding duplicates) from Google.