Rate Limiting is one of the ways that businesses attempt to prevent thieves from scraping websites for data. A rate limit allows an IP address a limited amount of searches in a given timeframe before access to the site is blocked. Users sharing a proxy server will easily reach rate limits. Proxy-server use is common for all users behind large corporate sites. Therefore, some legitimate users will not be allowed any searches.
What happens is, professionals who are scraping websites for data spread and distribute their searches over multiple IP addresses, so rate limiting does not affect them. Plus, a negative side effect of using rate limiting to stop scraping websites for data is that it often reduces legitimate site traffic. Sometimes site users and customers innocently exceed their rate limit of searches and mean no harm; thus webmasters must be careful not to block people from the site who are not looking for data scraping websites.