为什么内地居民去香港澳门属于出境，并且需要签注？

香港澳门同内地实行不一样的政治经济社会制度，主权回归了，特别行政区享有高度自治权。

中英联合声明和香港基本法都规定，内地居民进入香港需经批准，这是有法律依据的。

香港基本法22条：

For entry into the Hong Kong Special Administrative Region, people from other parts of China must apply for approval. Among them, the number of persons who enter the Region for the purpose of settlement shall be determined by the competent authorities of the Central People’s Government after consulting the government of the Region.

全国人大常委会还做过对该条的解释，同样具有法律效力：

For entry into the Hong Kong Special Administrative Region, people from other parts of China must apply for approval” mean as follows: People from all provinces, autonomous regions, or municipalities directly under the Central Government, including those persons of Chinese nationality born outside Hong Kong of Hong Kong permanent residents, who wish to enter the Hong Kong Special Administrative Region for whatever reason, must apply to the relevant authorities of their residential districts for approval in accordance with the relevant national laws and administrative regulations, and must hold valid documents issued by the relevant authorities before they can enter the Hong Kong Special Administrative Region. It is unlawful for people from all provinces, autonomous regions, or municipalities directly under the Central Government, including persons of Chinese nationality born outside Hong Kong of Hong Kong permanent residents, to enter the Hong Kong Special Administrative Region without complying with the appropriate approval procedure prescribed by the relevant national laws and administrative regulations.

实际上内地居民能否进入香港澳门或旅游或定居，完全取决于内地政府部门，香港澳门政府话语权约等于零。原因可参考其他答案。目前，香港市民希望限制内地居民来港的旅游人数，但主动权完全在内地政府部门。

其他回答都是从动机和原因角度出发，落实到手段上，就需要用法律来付诸实现了。所以本答案须以其他答案为背景。